[pm publish] add depfile support

Fixes a bug where incremental builds may not publish packages if there are
only contents / timestamp changes to the content files.

Test: build twice, observe null build. touch services.config, observe publish in next build.
Test: manual inspection of depfile to observe correct contents.
Change-Id: I8484e6d9aa36461f2da6aa3248a808e34771c9f3