go.mod: Add indirect exclusions/redirections from golang-protobuf upgrade

The golang-protobuf upgrade brought in new dependencies, which broke the
hermeticity of our go.mod redirections. Fix those up so they get
excluded or redirected to the locations in the tree.

Test: cd build/soong/cc; go test .
Change-Id: I012d11fad3b5509b213093e3c5889a27e641b868
diff --git a/go.mod b/go.mod
index 9a2ebcc..14444b3 100644
--- a/go.mod
+++ b/go.mod
@@ -8,4 +8,12 @@
 
 replace github.com/google/blueprint v0.0.0 => ../blueprint
 
+// Indirect deps from golang-protobuf
+exclude github.com/golang/protobuf v1.5.0
+
+replace github.com/google/go-cmp v0.5.5 => ../../external/go-cmp
+
+// Indirect dep from go-cmp
+exclude golang.org/x/xerrors v0.0.0-20191204190536-9bdfabe68543
+
 go 1.15