feat: add support for Label in nodejs_binary#entry_point

[manekinekko]

Closes #32

PR Checklist

Please check if your PR fulfills the following requirements:

• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature (please, look at the "Scope of the project" section in the README.md file)
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Documentation content changes
• Other... Please describe:

What is the current behavior?

nodejs_binary#entry_point was a string.

What is the new behavior?

nodejs_binary#entry_point is now a Label.

Does this PR introduce a breaking change?

• Yes
• No

The entry_point accepts a target's name as an entry point. If the target is a rule, it should produce the JavaScript entry file that will be passed to the nodejs_binary rule). For example:

       filegroup(
           name = "entry_file",
           srcs = ["workspace/path/to/entry/file"]
       )
       nodejs_binary(
         name = "my_binary",
         ...
         entry_point = ":entry_file",
       )

If the entry JavaScript file belongs to the same package (as the BUILD file), you can simply reference it by its relative name to the package directory:

        nodejs_binary(
          name = "my_binary",
          ...
          entry_point = ":file.js",
        )

⚠️ Downstream Rules That Need To Be Updated ⚠️

• rules_typescript: https://github.com/sfeir-open-source/rules_typescript/commit/9538466
• rules_sass: manekinekko/rules_sass@9862dfc

[manekinekko]

Please ignore the failing tests. This PR is in WIP.

[manekinekko]

@alexeagle you mentioned that we shouldn't actually use the expand_path_into_runfiles helpe and that Bazel has built-in function to get the path for a file. Which function did you have in mind?

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[manekinekko]

Blocked by #691

[manekinekko]

Status updates (with #691 applied locally):
✅./scripts/test_packages_all.sh
✅ ./scripts/test_examples_all.sh
✅ ./scripts/test_legacy_e2e_all.sh
❌ ./scripts/test_e2e_all.sh(because of firefox)

[gregmagolan]

Replaced by #777