The primary motivation is to support things like:
guix build guix --with-input=guile=guile-next
without triggering a rebuild of (@@ (gnu packages commencement)
guile-final) and similar things.
It is also consistent with package name resolution on the command line:
a package that cannot be named cannot be replaced.
* guix/packages.scm (package-input-rewriting/spec)[rewrite]: When P is
hidden, return it as-is.
* tests/packages.scm ("package-input-rewriting/spec, hidden package"):
New test.
* doc/guix.texi (Defining Package Variants): Update.
(Package Transformation Options): Update '--with-input' example.
