cmd/vet/all: vet using only source

cmd/vet/all: vet using only source

This simplifies the code and speeds it up.
It also allows us to eliminate some other TODOs;
those will come in a follow-up CL.

Running for the host platform, before:

real	0m9.907s
user	0m14.566s
sys	0m1.058s

After:

real	0m7.841s
user	0m12.339s
sys	0m0.572s

Running for a single non-host platform, before:

real	0m8.784s
user	0m15.451s
sys	0m3.445s

After:

real	0m7.681s
user	0m12.122s
sys	0m0.577s

Running for all platforms, before:

real	7m4.480s
user	8m43.398s
sys	1m15.683s

After:

real	4m37.596s
user	7m30.729s
sys	0m18.533s

It also makes my laptop considerably more
responsive while running for all platforms.

Change-Id: I748689fea0d2d4ef61aca2ce5524d03d8fafa5ca
Reviewed-on: https://go-review.googlesource.com/37691
Reviewed-by: Rob Pike <r@golang.org>