Logically the build needs to start treating a.Package as immutable,
since we might want to build a.Package multiple ways.
Record the built target in a.built instead.

Right now a.built is predictable ahead of time, but we want to
move toward satisfying some builds from a cache directory,
in which case a.built will point into the cache directory
and not be determined until action execution time.

There is probably more to do with shared libraries, but this
does not break what's there.

Change-Id: I941988b520bee2f664fd8cabccf389e1dc29628b
Reviewed-on: https://go-review.googlesource.com/69050
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>