Finished fix to allow install of packages with user-specified filenames.

Software installed from source, in cases where the URL doesn't contain the resulting filename (e.g. sourceforge /download/), don't install successfully using _get_install() because the existing wget call doesn't respect the server-supplied filename. This causes _get_expected_file() to throw an error as it can't decide how to unpack the downloaded file. This patch finishes some (already begun) fixes to allow the user of _get_install() to specify the resulting downloaded file name so that it is both correct and known.