SwiftPM plugin: allow file glob patterns in `protoFiles` file list

[klundberg]

As mentioned in another issue, we have our proto files integrated using submodules. We want to generate code for proto files in specific directories in this submodule. in the protoc command, we can do this with glob patterns, and it'd be great to be able to support this in the plugin so that we don't need to have a large list of files, if possible.

Our use case requires a pattern that looks like this:

submodules/**/feature/*.proto

Alternatively, if glob's aren't possible, adding a directory that holds proto files would allow the config to be more lean and easier to maintain.

[FranzBusch]

This is something that we would like to have but currently the SPM plugin interface is not providing any functionality for this. I went ahead and opened an issue over in the SPM repo to track this: swiftlang/swift-package-manager#5804

[klundberg]

Great, thank you!

[thomasvl]

Closing this since I think at this point we'd rather see SwiftPM get the support directly instead of something custom here.