cmd/go: do not build test packages unnecessarily during go vet
Vet needs export data for the imports of the package it is analyzing.
Vet does not need export data for the package itself, since vet will
do its own type checking. Assuming that vet is just as good as the compiler
at detecting invalid programs, don't run the compiler unnecessarily.
This especially matters for tests without external test files or for
which the external test files do not import the test-augmented original
package. In that case, the test-augmented original package need not
be compiled at all.
Cuts time for 'go clean -cache && go vet -x cmd/compile/internal/ssa'
from 7.6r 24.3u 2.8s to 3.5r 8.5u 1.9s, by not running the compiler
on the augmented test package.
There is still more to be done here - if we do need to build a
test-augmented package, we rerun cgo unnecessarily.
But this is a big help.