This moves the fileset down to the base cache, the overlays down to the session
and stores the environment on the view.
packages.Config is no longer part of any public API, and the config is build on
demand by combining all the layers of cache.
Also added some documentation to the main source pacakge interfaces.
Change-Id: I058092ad2275d433864d1f58576fc55e194607a6
Reviewed-on: https://go-review.googlesource.com/c/tools/+/178017
Run-TryBot: Ian Cottrell <iancottrell@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rebecca Stambler <rstambler@golang.org>
7927dbab1b