cmd/gofix: should warn about exp, old, deleted

[robpike]

gofix should print a warning if it sees imports of these packages:

to exp:

ebnf
go/types
http/spdy

old:
old/netchan
old/regexp
old/template

deleted:

container/vector
exp/datafmt
go/typechecker
try

[gopherbot]

Comment 1 by tux21b:

I would like to give it a try.
http://golang.org/cl/5592043/

[rsc]

Comment 2:

This issue was closed by revision 8781536.

Status changed to Fixed.