Proto import file globs

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.

Start at the --protos option and trace the existing proto file lookup and import handling. Check how multiple proto paths are currently prioritized; done means glob patterns work for the shown path forms and preserve the stated lookup priority when names conflict.

Written by the indexing model from the issue text.