cmd/go: vet: pass non-.go files to vet tool

The "gofiles" cache entry has been renamed "srcfiles", and it includes
non-Go files (.s, .c, .cxx) that belong to the package. It does not
include raw cgo files.

Added regression test.

Fixes #27665

Change-Id: I4884fe9b4f823f50705f8c2d357a04a8e567734f
Reviewed-on: https://go-review.googlesource.com/c/148904


Reviewed-by: Bryan C. Mills <bcmills@google.com>