x/tools/cmd/goimports: make zstdlib.go aware of appengine packages

[gopherbot]

by lesv@angeltech.com:

It would be nice if zstdlib.go which appears to be autogenerated would know about
appengine files.  It would make goimports work better w/ AppEngine files as well as
other tools.

What does 'go version' print?

What steps reproduce the problem?
If possible, include a link to a program on play.golang.org.

1.
2.
3.

What happened?

What should have happened instead?

Please provide any additional information below.

[minux]

Comment 1:

Labels changed: added release-go1.5, repo-tools.

[seankhliao]

I think this is no longer necessary with the new appengine environments