Proto import file globs

[kriswuollett]

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Perhaps to satisfy some use cases like error handling conventions in #238, could file globs be supported? Assuming someone doesn't have file naming it would be a lot easier to use something like --protos .:my_org/**/*.proto --protos ./third_party:**/*.proto than listing individual proto files to do things like reflect on custom error details. The order of protos parameter would determine the lookup priority if there are any naming conflicts.

My use case would be checking in all third party protos like those from https://github.com/googleapis/api-common-protos into ./third_party and be able to use the convention where things like FieldViolation is returned in gRPC errors.