Documentation of build.Import says:

	// If the path is a local import path naming a package that can be imported
	// using a standard import path, the returned package will set p.ImportPath
	// to that path.
	// ...
	// If an error occurs, Import returns a non-nil error and a non-nil
	// *Package containing partial information.

That behavior was previously untested, and broken by change in CL 33158.

Fix that by avoiding returning early on error for local import paths.
First, gather partial information, and only then check that the p.Dir
directory exists.

Add tests for this behavior.

Fixes #19769.
Fixes #20175 (duplicate of #19769).
Updates #17863.

Change-Id: I169cb35291099d05e02aaa3cb23a7403d1cc3657
Reviewed-on: https://go-review.googlesource.com/42350Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>