When calling build.Import, normally, an error is returned if the
directory doesn't exist. However, that didn't happen for local
import paths when build.FindOnly ImportMode was used.

This change fixes that, and adds tests. It also makes the error
value more consistent in all scenarios where it occurs.

When calling build.Import with a local import path, the package
can only exist in a single deterministic directory. That makes
it possible verify that directory exists earlier in the path,
and return a "cannot find package" error if it doesn't.
Previously, this occurred only when build.FindOnly ImportMode
was not set. It occurred quite late, after getting past Found
label, to line that calls ctxt.readDir. Doing so would return
an error like "no such file or directory" when the directory
does not exist.

Fixes #17863.
Updates #17888 (relevant issue I ran into while working on this CL).

Change-Id: If6a6996ac6176ac203a88bd31419748f88d89d7c
Reviewed-on: https://go-review.googlesource.com/33158Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>