Feature request: Add folder reference support (PBXFileReference with folder semantics)

[meyers-oai]

rules_xcodeproj currently models most directories as Xcode groups. Have we considered support for generating Xcode folder references (aka blue folders) for selected paths? Assuming it's most common to specify sources as a glob (i.e. 'Sources/**/*.swift') Using folder references would allow Xcode to immediately show the correct state after switching branches or adding/removing/moving files. Currently we need to re-generate the project.

[adincebic]

@meyers-oai I have it on my todo list since forever. However it may be trickier than one might think due to how bazel handles globs in analysis phase. I believe @brentleyjones expressed that concern a while ago on Bazel Slack.