Fix issue due to go/build.ImportDir change response on not found dir

[mattfarina]

In go 1.9 there was a change to the way ImportDir, or rather Import
with a local "." package, responded when not found. Previously,
the error was an OS not found error that could be detected by
os.IsNotExist. Now the error is a custom go error using the format:

fmt.Errorf("cannot find package %q in:\n\t%s", path, p.Dir)

This change looks for both cases. For Go 1.9 is checks of it doesn't
exist because there are similar errors in go/build.

For more detail on the Go side of this see golang/go#21923

[mattfarina]

@sdboyer you might be interested in this error. Or, maybe you already have this one down.

[sdboyer]

ah, thanks for the heads-up! we moved away from directly using build.ImportDir and rely directly on go/parser now, so we're set.