Revert "cmd/stringer: use source importer when available"
This reverts CL 40403.
The idea is to avoid type-checking and use just parsing, which should be
enough for stringer.
Separately reopening golang/go#10249 because the original change closed that issue,
but the change is itself causing other problems as described in the discussion
at golang/go#25650.
This reversion restores the old behavior of stringer and will be followed
with other fixes if they can be worked out.
Change-Id: I8404d78da08043ede1a36b0e135a3fc7fdf6728d
Reviewed-on: https://go-review.googlesource.com/121884
Reviewed-by: Ian Lance Taylor <iant@golang.org>