go/tools/bazel: refactor runfiles functionality

[jayconrod]

• Refactored Runfile, FindBinary, and other functions. Everything now
  calls a single initialization function through sync.Once.
• Added ListRunfiles. This will be used to re-create trees of
  runfiles, which will be needed for the new version of bazel_test.
• Moved and rewrote tests in tests/core/runfiles.

Fixes #2031
Fixes #1728

[alexeagle]

hey there, I think this change broke rules_typescript
https://github.com/bazelbuild/rules_typescript/blob/master/devserver/runfiles/runfiles.go
it seems that Runfile("build_bazel_rules_nodejs/packages/protractor/test/protractor-2/app.js") used to work (starts with workspace name) but now it must be Runfile("packages/protractor/test/protractor-2/app.js") (no workspace name)?
Does that sound right?

[jayconrod]

Are you able to switch to not using the workspace name? Runfile should only accept relative paths from the workspace root, but it was (and is still) way too permissive on the input path.

I'm sorry for the break though. Runfile was originally written by a few external contributors, and it wasn't adequately documented or tested. I needed to refactor it and make it work on Windows to make progress, but it was hard to make any change without subtly breaking something.

[alexeagle]

Is it really the right interface to accept a path with no workspace name? If I have a file index.js in the local workspace runfiles and also in an external one, I would have no way to locate the external index.js in the runfiles.

[jayconrod]

It depends on the situation, but no, Runfile is a poor interface.

ListRunfiles provides the most complete information. I'd recommend using that if files may be in multiple workspaces or if you're using a mix of short paths and regular paths.

If you can pass the location directly through a build file with something args = ["$(location :label)"], that may let you avoid this library altogether.

[alexeagle]

I'm not ready to figure out that refactoring right now so I'll just patch out this commit from our rules_go http_archive fetch. One day the merge conflict will bring someone back to this thread. Maybe me! :)