go.tools: create subrepository

This subrepo is the ultimate home for some of the experimental components
being developed in go.exp now, and also existing pieces such as go vet.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/9481043

README

@@ -1,16 +1,7 @@
-This is an empty Go subrepository.  To create a new subrepository,
-visit http://code.google.com/p/go/adminSource and under repositories,
-type the new name, check [x] Clone contents and select [empty] as
-the one you want to clone.
+This subrepository holds the source for various packages and tools that support
+the Go programming language.
 
-Then execute:
-	go get code.google.com/p/go.newrepo
-	cd $(go list -e -f '{{.Dir}}' code.google.com/p/go.newrepo)
+Although at least some of the tools themselves will be included in binary Go distributions,
+the packages from which they are built are of little interest to most Go programmers.
 
-The go get will complain about not finding source code, but it will
-successfully check out the repository.
-
-Edit the README (this file) to describe the new subrepository, and then
-use the usual hg change, mail, submit to send in the change.
-You will need to follow http://golang.org/doc/contribute.html#Code_review
-to enable the Code Review extension (pointing into your Go root).
+To submit changes to this repository, see http://golang.org/doc/contribute.html.