cmd/go: refactor package references in Builder.build

cmd/go: refactor package references in Builder.build

Refactor references to "a.Package" in Builder.build to be consistent,
e.g. all via the "p" helper variable. No change in functionality, this
is just intended to make the code more readable/consistent (since
prior to this point we had a random mix of "p" and "a.Package").

Change-Id: Ifea5ff9b314c39a0cf6e688511907d08cc56c603
Reviewed-on: https://go-review.googlesource.com/c/go/+/424819
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Than McIntosh <thanm@google.com>