Add a pbswift_files output group and use PathToUnderscores

[steeve]

Add a new pbswift_files output group in order to consume the generated pbswift files in macros for instance

Switch from FileNaming=FullPath to FileNaming=PathToUnderscores When compiling pbswift files, the linker complains if two files have the same name. This fixes it.

[tinder-maxwellelliott]

@steeve is there any way to accomplish the PathToUnderscores change via a feature flag or something? Looking for options here as well since I of course have duplicated proto filenames....