cmd/go: improve error message when unable to detect symlinked packages during go install

[krisnova]

V 1.6.2 Darwin/AMD64

filepath.Walk does not (and will not) respect traversing symlinks. (See #17451)

Can we improve the error message when a user attempts to run go install on a package that is a symlink? Right now the error message is "warning: %q matched no packages\n".

That does not offer the user insight into why the package did not build as expected.

[bradfitz]

Kinda documentation. But also code. For Go 1.8 if trivial.

[gopherbot]

CL https://golang.org/cl/46425 mentions this issue.