imports: add configuration mechanism to exclude directories

Each $GOPATH entry may have a file $GOPATH/src/.goimportsignore which
may contain blank lines, #comment lines, or lines naming a directory
relative to the configuration file to ignore when scanning.  No
globbing or regex patterns are allowed.

Updates golang/go#16367 (goimports speed)
Fixes golang/go#16386 (add mechanism to ignore directories)

Change-Id: I8f1a88ae6c4d0ed3075444d70aec3e2228c5ce6a
Reviewed-on: https://go-review.googlesource.com/24971
Reviewed-by: Rob Pike <r@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
3 files changed