Everything got a bit tangled together in b.action1.
Try to tease things apart again.

In general this is a refactoring of the existing code, with limited
changes to the effect of the code.

The main additional change is to complete a.Deps for link actions.
That list now directly contains all the inputs the linker will attempt
to read, eliminating the need for a transitive traversal of the entire
action graph to find those. The comepleteness of a.Deps will be
important when we eventually use it to decide whether an cached
action output can be reused.

all.bash passes, but it's possible I've broken some subtety of
buildmode=shared again. Certainly that code took the longest
to get working.

Change-Id: I34e849eda446dca45a9cfce02b07bec6edb6d0d4
Reviewed-on: https://go-review.googlesource.com/69831
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>