Move third_party code under third_party/src so it can be used in $GOPATH.

hooks/prepare-commit-msg

@@ -1,7 +1,7 @@
 #!/bin/bash
 
 errors=0
-for file in $(git diff --cached --name-only | grep "\.go"); do
+for file in $(git diff --cached --name-only --diff-filter ACM | grep "\.go" | grep -v "third_party"); do
   diff="$(git show ":${file}" | gofmt -s -d)"
   if [[ -n "$diff" ]]; then
     echo "# *** ERROR: *** File ${file} has not been gofmt'd." >> $1