fix: use tree artifacts via copy_directory with exports_directories_only

[gregmagolan]

Fixes the issue with exports_directories_only on RBE.

Removes has_directories from ExternalNpmPackageInfo since it is expected that source directories
are no longer used. This simplified npm_umd_bundle.bzl impl which was using this to filtering; it can now
just check File.is_directory since source directories are no longer passed in.

[gregmagolan]

this is no longer needed since we no longer have this case as exports_directories_only now produced proper individually linkable js_library targets with package names instead of a js_library with a source directory and its package name set to the special case $node_modules_dir$

[alexeagle]

Had a quick scan over it, looking good. LMK when it's green and I'll do a more careful review

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[alexeagle]

@googlebot I consent.

[matthewjh]

@gregmagolan what's the motivation behind this change?

[alexeagle]

It fixes the exports_directories_only feature to work with RBE and with --nobuild_runfile_links

The feature originally depended on Bazel accidentally not noticing that inputs were directories, which happened to work in almost all cases. Now we make them TreeArtifacts which unfortunately bazel can't produce as InputArtifacts so a copy operation is needed.

[matthewjh]

Thanks @alexeagle. I am about to set up RBE for my org. From what you say it sounds like I will have to turn off exports_directories_only for things to work, until this lands? Or does it only not work with both exports_directories_only and --nobuild_runfile_links?

[alexeagle]

Doesn't work with either, is my understanding