fix(transpile): don't throw away untranspiled srcs

[alexeagle]

Note, this still needs some analysis testing, and an example that makes it load-bearing.

Also it's worth thinking about the correct behavior in the non-transpiler use case, though that's a bit more subtle.

[alexeagle]

filegroup srcs are a critical design flaw here. thinking about a simpler approach.

[alexeagle]

@gregmagolan and I don't remember the context of this one. It looks like our goal was to permit sass source files to ts_project, but I don't see an issue filed for it. I guess if it's important, it will come up again and we can search for this WIP and try to make progress on the thorny design issue