Expose source-files and project structure

[njlr]

In order to build Bazel rules for Fable, we need access to the F# source-files and target dependency graph. This information can be used to generate fsproj files that the Fable CLI tool can understand.

This PR exposes these things, but in a generic way, so that it can be used for multiple purposes.

I was able to build a proof-of-concept set of rules for Fable on top of this. However, before going further I would like feedback on these changes, since I hope to eventually have them merged into rules_dotnet.

Thanks!

Related: https://github.com/bazel-contrib/rules_dotnet/issues/315