all: make .proto file names relative to module root

Change the protoc flags such that when one of our test .proto files
imports another, the filename is consistently specified relative to the
module root.

Change-Id: I690282795cef23347c8794c1c6357e4fe9560d8a
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/217762
Reviewed-by: Joe Tsai <joetsai@google.com>
diff --git a/internal/testprotos/irregular/irregular.go b/internal/testprotos/irregular/irregular.go
index da5c396..a663e87 100644
--- a/internal/testprotos/irregular/irregular.go
+++ b/internal/testprotos/irregular/irregular.go
@@ -100,7 +100,7 @@
 	return file
 }()
 
-func file_irregular_irregular_proto_init() { _ = fileDesc }
+func file_internal_testprotos_irregular_irregular_proto_init() { _ = fileDesc }
 
 const descriptorText = `
   name: "internal/testprotos/irregular/irregular.proto"