Allow plugins/presets to indicate external dependencies

[nicolo-ribaudo]

Q	A
Fixed Issues?	Fixes #8497, closes #11741
Patch: Bug Fix?
Major: Breaking Change?
Minor: New Feature?	Yes
Tests Added + Pass?	Yes
Documentation PR Link
Any Dependency Changes?
License	MIT

This PR is an alternative to #11741, addressing the concerns I explained in my review:

• it avoids storing external dependencies as global state, but tracks them locally for each babel.transform call. The caller is then free to merge/overwrite the external dependencies between different calls as it wish.
• since external dependencies are babel.transform-local, api.addExternalDependency only needs to take the dependency path as a parameter.

Additionally, I made a few other design choices:

• @babel/core does not resolve external dependencies: they can be any string you want. This makes it possible, for example, to also store URLs or pointers to a virtual fs (as long as the caller knows how to handle them).
• When a plugin calls api.addExternalDependency, it must also explicitly configure the cache (for example, with cache.invalidate or cache.using). This is because when you have external dependencies you rely on a side communication channel that Babel does not control, so you have to manually invalidate the cache when it changes.

@vedantroy thanks for your awesome work in #11741! I made these changes by myself because I noticed you asked us to do so and I changed almost all the code you initially wrote, but your original design (especially regarding @babel/cli) helped me a ton figuring out what is the best way to integrate this feature in Babel 😄

[babel-bot]

Build successful! You can test your changes in the REPL here: https://babeljs.io/repl/build/50985/

[goatandsheep]

Just a heads up: with this design, since I maintain a babel plugin that checks .env files, I don't think this will help since I can't tell it to invalidate constantly. Adding external dependencies is a feature that really helps prevent excessive cache clearing

[nicolo-ribaudo]

You can do this:

function plugin(api) {
  const dotEnvFilename = `${process.cwd()}/.env`;

  const env = api.cache.using(() => fs.readFileSync(dotEnvFilename, "utf8"));
  api.addExternalDependency(dotEnvFilename);
  
  return {
    visitor: {}...
  }
}

The api.addExternalDependency(dotEnvFilename) line tells to whatever is calling Babel (for example, @babel/cli) that it needs to be re-triggered when .env changes; const env = api.cache.using(() => fs.readFileSync(dotEnvFilename, "utf8")); reads the .env file and re-instantiates the plugin whever it changes.

[JLHwung]

Suggested change

	`+statSync(filepath).mtime)\` or \`api.cache.never()\`\n` +
	`statSync(filepath).mtimeMs)\` or \`api.cache.never()\`\n` +

[JLHwung]

I think it suffices to use TS ReadOnlyArray for externalDependencies, since eventually they would be added to a Set.

[sosukesuzuki]

I commented some nits!