Allow AST plugins' output be cached based on external files

[chriseppstein]

In order for certain AST plugins to be cached properly, they may need to include other files into their cache key.

This is useful when you'd like to change the compiled template output based on the contents of a component.css file (which is something that css-blocks would need). Prior to this PR the only way to do this was to have the AST plugin completely opt out of caching (by returning a unique value for its cache key) which obviously has fairly severely negative implications for overall build performance, but is not even enough! Editing the .hbs file would gather any .css file changes, but editing the related .css would still never cause the .hbs file to be recompiled.

[chriseppstein]

Tests are failling in windows, but passing in os-x. I've setup a dev environment in window and can reproduce the issue now. Will have a fix soon.

[chriseppstein]

Tests are green. I had to disable some of the legacy ember version tests for ast plugins because there was no way to unregister plugins once registered.

[chriseppstein]

I'd like to squash all these commits down to just 1 commit prior to landing the code.

[rwjblue]

Overall looks very good, only one relatively small change needed then we should be good to release.

Thanks for working through this!

[rwjblue]

LGTM, thanks for your hard work on this!

[rwjblue]

Gonna leave merge + release for the AM (never merge and release just before bed 😝).

@stefanpenner - Lemme know if you have any objections...

[chriseppstein]

🎉 Thanks @rwjblue & @stefanpenner for helping landing this!

[Turbo87]

I just saw this PR in the changelog but the title and description do not really describe what it does or why it's an enhancement. Could you elaborate on the purpose of this PR?

[rwjblue]

@Turbo87 - Updated title and description to be more descriptive.

[Turbo87]

thanks 🙏