Replace the name Environment with Context, as discussed in #47916. Along
the way, fix some stale or inaccurate comments.

The Environment type remains temporarily as an alias for Context, to
allow the x/tools Trybot to pass until dependency on types.Environment
can be removed.

Updates #47916

Change-Id: Iffd069ab0e8adebf4207c8f8891468a64d32b7cc
Reviewed-on: https://go-review.googlesource.com/c/go/+/353089


Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
Reviewed-by: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>