packages: 'specification->package+output' distinguishes "no output specified".

packages: 'specification->package+output' distinguishes "no output specified".

Until now the caller couldn't tell the different between a spec like
"foo:out" and one like "foo".  This change allows users to distinguish
between these two cases.

* gnu/packages.scm (specification->package+output): Disable output
membership test when OUTPUT = #f and SUB-DRV = #f.
* tests/packages.scm ("specification->package+output")
("specification->package+output invalid output")
("specification->package+output no default output")
("specification->package+output invalid output, no default"): New tests.