packages: The 'source' can be any lowerable object.

* guix/packages.scm (expand-input): Use 'struct?' instead of 'origin?'
when matching SOURCE.
(package-source-derivation): Use 'lower-object' instead of
'origin->derivation'.
* tests/packages.scm ("package-source-derivation, local-file"): New
test.
* doc/guix.texi (package Reference): Update 'source' documentation
accordingly.