Return a list of files that were read from loadPartialConfig

[devongovett]

This adds a files list to the return value of babelCore.loadPartialConfig which lists all files that were read in the process of loading the config. Some of these are currently returned in the babelrc, babelignore, and configFile fields, but other files such as those that were extended from another config are not returned, making it difficult for other tools that consume babel (e.g. Parcel, webpack) to properly watch these extended configs and invalidate caches. The files key should solve this as it includes all files that influenced the config, not just the top-level ones.

In addition, I've added a showIgnoredFiles option to loadPartialConfig which always returns a PartialConfig result even when the file is ignored, rather than returning null. This allows tools to properly invalidate caches when whatever config file that marked the compiled file as ignored changes. An isIgnored boolean is returned as part of the PartialConfig result to indicate that the file is ignored. Given that changing the null result to return a value would be a breaking change, this is behind an option. Open to better name suggestions though! 😉

This is really important for us in Parcel so that we can get proper caching working with Babel. At the moment there's really no good way to do that without support from Babel itself, or reinventing all of its config loading. This should also help other tools like babel-loader - found a todo comment for it. 😄

Totally open to alternative suggestions. Really made this to get the conversation started.

Q	A
Fixed Issues?
Patch: Bug Fix?
Major: Breaking Change?
Minor: New Feature?	Yes
Tests Added + Pass?	Yes
Documentation PR Link	babel/website#2333
Any Dependency Changes?
License	MIT

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[devongovett]

Anything I can do to move this forward? 😄

[JLHwung]

Look good to me. If we get enough approvals, will need to update https://babeljs.io/docs/en/babel-core#loadpartialconfig

[devongovett]

I'm happy to send a PR for that!

[devongovett]

babel/website#2333

[nicolo-ribaudo]

I'm merging this to feat-7.12.0/config-loading-updates, so that I can rebase #11689.

[nicolo-ribaudo]

@devongovett @JLHwung @existentialism @kaicataldo When I asked for fileHandling to be a string so that it can allow three values (transpile, ignored and unsupported), I was thinking that I could use it for 9114ffc (#12151): my idea was to load the config for a file, check if it's "ignored" or "unsupported", and act accordingly.

However, while implementing it I realized that it's not possible because loadPartialConfig does not resolve presets, and thus it doesn't have info about the supported extensions.

I found another way of detecting ignored vs unsupported: if loadPartialConfig returns null it's ignored; otherwise if transformFile returns null it's unsupported.

For this reason, I propose to change back the design of this PR to isIgnored: boolean rather than fileHandling: string. I can work on it this weekend if you agree, and I'm sorry for having asked @devongovett to do some unnecessary work.

[JLHwung]

@nicolo-ribaudo I don't have strong preference here. But I believe

I found another way of detecting ignored vs unsupported: if loadPartialConfig returns null it's ignored; otherwise if transformFile returns null it's unsupported.

should be documented somewhere. If fileHandling: StatusEnum is more friendly to API users, we can go with current design.

[nicolo-ribaudo]

Sure, I'll document it alongside with the extensions option docs!