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/test/ext.proto b/internal/testprotos/test/ext.proto
index eec689c..9ff1dd6 100644
--- a/internal/testprotos/test/ext.proto
+++ b/internal/testprotos/test/ext.proto
@@ -6,7 +6,7 @@
 
 package goproto.proto.test;
 
-import "test/test.proto";
+import "internal/testprotos/test/test.proto";
 
 option go_package = "google.golang.org/protobuf/internal/testprotos/test";