Prevent optional dependency Chokidar from loading when not watching

[eklingen]

Summary

Proposed change:

Since Chokidar is an optional dependency, and relatively heavy, it would be best to defer requiring it until the watcher is actually needed. This gives a slight speed increase when using FileSystemResolver or NodeResolver with the watch option set to false, unless the end-user has already loaded Chokidar.

Triggering require() twice when using both loaders does not have a big performance impact because of the module cache. This is an internal change only. I have not written tests, since these would depend upon module.cache[] side-effects to run.

Checklist

I've completed the checklist below to ensure I didn't forget anything. This makes reviewing this PR as easy as possible for the maintainers. And it gets this change released as soon as possible.

• Proposed change helps towards purpose of this project.
• Documentation is added / updated to describe proposed change.
• Tests are added / updated to cover proposed change.
• Changelog has an entry for proposed change (if user-facing fix or feature).

[codecov-io]

Codecov Report

Merging #1250 into master will decrease coverage by 0.06%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #1250      +/-   ##
==========================================
- Coverage    89.6%   89.53%   -0.07%     
==========================================
  Files          22       22              
  Lines        3020     3020              
==========================================
- Hits         2706     2704       -2     
- Misses        314      316       +2

Impacted Files	Coverage Δ
nunjucks/src/node-loaders.js	65.67% <0%> (-2.99%)	⬇️

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[eklingen]

As you can see from this trace, a require('nunjucks') call takes (on my system) ~177ms, of which ~160ms is spent on importing chokidar, even when watch is set to false. With this change, nunjucks takes only ~17ms to load. Every little bit helps. ;)

  177 ms            └─ node_modules/nunjucks/index.js (10x)
  174 ms                └─ node_modules/nunjucks/src/environment.js (8x)
    9 ms                    ├─ node_modules/nunjucks/src/compiler.js (6x)
    4 ms                    │   └─ node_modules/nunjucks/src/parser.js (3x)
  162 ms                    └─ node_modules/nunjucks/src/loaders.js
  161 ms                        └─ node_modules/nunjucks/src/node-loaders.js
  160 ms                            └─ node_modules/chokidar/index.js (10x)
  107 ms                                ├─ node_modules/anymatch/index.js (3x)
  106 ms                                │   └─ node_modules/micromatch/index.js (7x)
   80 ms                                │       ├─ node_modules/braces/index.js (7x)
   13 ms                                │       │   ├─ node_modules/to-regex/index.js (4x)
    4 ms                                │       │   │   ├─ node_modules/safe-regex/index.js
    4 ms                                │       │   │   └─ node_modules/to-regex/node_modules/define-property/index.js
    5 ms                                │       │   ├─ node_modules/braces/lib/compilers.js
    5 ms                                │       │   ├─ node_modules/braces/lib/parsers.js (2x)
   56 ms                                │       │   └─ node_modules/braces/lib/braces.js (4x)
   54 ms                                │       │       └─ node_modules/snapdragon/index.js (5x)
   28 ms                                │       │           ├─ node_modules/base/index.js (6x)
   13 ms                                │       │           │   ├─ node_modules/cache-base/index.js (4x)
    8 ms                                │       │           │   │   └─ node_modules/unset-value/index.js
    6 ms                                │       │           │   └─ node_modules/class-utils/index.js (3x)
   14 ms                                │       │           └─ node_modules/snapdragon/lib/compiler.js
   13 ms                                │       │               └─ node_modules/snapdragon/lib/utils.js (2x)
    4 ms                                │       │                   └─ node_modules/source-map-resolve/lib/source-map-resolve-node.js
   17 ms                                │       └─ node_modules/micromatch/lib/compilers.js (2x)
    8 ms                                │           └─ node_modules/extglob/index.js (4x)
    4 ms                                │               └─ node_modules/extglob/lib/compilers.js
   49 ms                                └─ node_modules/chokidar/lib/fsevents-handler.js
   45 ms                                    └─ node_modules/fsevents/fsevents.js (5x)
   41 ms                                        └─ node_modules/fsevents/node_modules/node-pre-gyp/lib/node-pre-gyp.js (6x)
   27 ms                                            ├─ node_modules/fsevents/node_modules/npmlog/log.js (6x)
   12 ms                                            │   ├─ node_modules/fsevents/node_modules/are-we-there-yet/index.js (3x)
   11 ms                                            │   │   └─ node_modules/fsevents/node_modules/are-we-there-yet/tracker-group.js
    7 ms                                            │   │       └─ node_modules/fsevents/node_modules/are-we-there-yet/tracker-stream.js (3x)
    6 ms                                            │   │           └─ node_modules/fsevents/node_modules/readable-stream/readable.js (5x)
   12 ms                                            │   └─ node_modules/fsevents/node_modules/gauge/index.js (8x)
    9 ms                                            │       └─ node_modules/fsevents/node_modules/gauge/plumbing.js (2x)
    6 ms                                            │           └─ node_modules/fsevents/node_modules/gauge/render-template.js (6x)
    8 ms                                            ├─ node_modules/fsevents/node_modules/node-pre-gyp/lib/util/napi.js (2x)
    7 ms                                            │   └─ node_modules/fsevents/node_modules/rimraf/rimraf.js
    4 ms                                            └─ node_modules/fsevents/node_modules/node-pre-gyp/lib/pre-binding.js (5x)

[fdintino]

LGTM. I've merged this. Thanks!

[eklingen]

Cool! 👍