cmd/go: avoid a few symlink-induced errors in internal and vendor checks

cmd/go: avoid a few symlink-induced errors in internal and vendor checks

This CL expands symlinks only when an error would be reported otherwise.
Since the expansions are only on error paths, anything that worked yesterday
should still work after this CL.

This CL fixes a regression from Go 1.5 in "go run", or else we'd probably
postpone it.

Changing only the error paths is meant as a way to reduce the risk of
making this change so late in the release cycle, but it may actually be
the right strategy for symlinks in general.

Fixes #14054.

Change-Id: I42ed1276f67a0c395297a62bcec7d36c14c06404
Reviewed-on: https://go-review.googlesource.com/19102
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>