Instead of only checking packages that are already listed in pkgDeps,
apply deps checks to all standard library packages.
To avoid slowing testing down too much, instead of running "go list
std" in a subprocess like cmd/api or cmd/dist, this test manually
walks the GOROOT src directory to enumerate packages.
Timings on an HP Z620 using linux/amd64:
short full
before 0.092s 4.880s
after 0.137s 5.104s
Additionally, a handful of packages that were previously unchecked are
now listed, along with their current dependencies. These should
probably eventually be moved elsewhere and assigned appropriate
allowable-dependency sets. For now, they've been grandfathered in by
simply assigning them their current dependencies, so that followup CLs
can review them individually as appropriate.