cmd/goimports: reuse cached state
cmd/goimports: reuse cached state

The internal imports API allows the user to control the lifetime of
caches, via the ProcessEnv object. Change the goimports command to use
the same cache for its lifetime. This should speed up goimports -w *.go
dramatically in cases where imports need to be added.

Change-Id: I01e3531ad53b038896435474ac9a8be97d5f3c10
Reviewed-on: https://go-review.googlesource.com/c/tools/+/175448
Run-TryBot: Heschi Kreinick <e89ab2b172855aa0ad01c40c4d8a3535a61ac68a@google.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/tools,golang/tools,golang/tools,golang/tools,golang/tools