cmd/go/internal/modload: fail if no package was found in local module
Changing the Import function to return a PackageNotInModuleError if no
package was found in a local module. This replacing the vague message
"missing dot in first path element" you get today with much more friendly
one - "module was found, but does not contain package".
Fixes #35273
Change-Id: I6d726c17e6412258274b10f58f76621617d26e0a
Reviewed-on: https://go-review.googlesource.com/c/go/+/203118 Reviewed-by: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>