hammer.git: add list target

Add a target to list all packages.
setup: deprecate "www" PKG_TYPE

"www" was a silly type name for tarballs fetched from the web.
Let's assume such format by default, unless PKG_TYPE is set to "git".
setup: add a check for an empty PKG_URL

Add a check for an empty PKG_URL.
setup: allow build scripts to override upstream/packages

Allow build scripts to override upstream/packages by providing PKG_URL.