Accept rules_pkg manifests as input for image_layer

[peakschris]

We have extensive use of rules_pkg's pkg_files and pkg_filegroup macros through our codebase and appreciate them for their structured approach to composing layers. It already has a provider that allows archive content to be declared without manifesting it to disk. Could rules_img read this provider from within image_layer or via a new layer rule? Direct interop would help us in the interim until your proposal (#74) reaches wide adoption.

Thanks!

[malt3]

I'm slightly hesitant. Mostly because this requires adding a bazel_dep on rules_pkg, which depends on rules_python and plenty of other things.
My plan for fsmanifestinfo is for it to be a drop-in replacement for PackageFilesInfo (the provider returned by pkg_files) in a clean module that has no dependencies.
I'll think about this. Maybe the dependency is not an issue in practice, or we find a way to move all providers out of rules_pkg?

[peakschris]

I hear you on not wanting to add lots of transitive dependencies. We are avoiding rules_python too where we can. I guess there are some options:

1. extend rules_pkg to emit both PackageFilesInfo and fsmanifestinfo
2. extend rules_pkg to emit either PackageFilesInfo or fsmanifestinfo [configurable per codebase/rule]
3. move providers.bzl out of rules_pkg and update rules_img to consume it

@aiuto any thoughts?