cmd/go: fix error for empty packages referenced with relative paths
'go build' now reports a more useful error when a relative path on the
command line points to a directory that doesn't exist or a directory
without .go files. Errors are generated by go/build.Context.ImportDir
instead of a vague call to base.Fatalf in modload.
Fixes #35414
Change-Id: I2642230c5e409107b98bb6d6c3a484d8d25b4147
Reviewed-on: https://go-review.googlesource.com/c/go/+/206902
Run-TryBot: Jay Conrod <jayconrod@google.com> Reviewed-by: Bryan C. Mills <bcmills@google.com>