build: 'guix-manual.*.po' no longer depends on 'guix-manual.pot'.

build: 'guix-manual.*.po' no longer depends on 'guix-manual.pot'.

That dependency was causing a failure when building from a fresh
checkout since commit d60225d5caabfb6409e3277c0512cd4c0fa63d72 because
'guix-manual.pot' was no longer around and there's no rule to make it.

This commit replaces the 'guix-manual.%.po' target by a phony target,
'doc-po-update-%', without any dependency.

* po/doc/local.mk ($(srcdir)/po/doc/guix-manual.%.po): Rename to...
(doc-po-update-%): ... this.  Remove dependencies.
Change how 'lang' is computed; compute $output and $input, and replace
occurrences of $@ and $< with those.
(doc-po-update): Adjust accordingly.