cmd/api: eliminate duplicate package import work

On my Mac, cuts the API checks from 15 seconds to 6 seconds.
Also clean up some tag confusion: go run list-of-files ignores tags.

R=bradfitz, gri
CC=golang-dev
https://golang.org/cl/12699048

src/cmd/api/run.go

@@ -1,9 +1,9 @@
-// +build from_src_run
-
 // Copyright 2013 The Go Authors.  All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build ignore
+
 // The run program is invoked via "go run" from src/run.bash or
 // src/run.bat conditionally builds and runs the cmd/api tool.
 //