Change syntax of '--with-source' flag to match syntax of '--with-commit' flag?

[<pkill9@runbox.com>]

The '--with-commit' flag changes the source used to build a package, the syntax takes the form of '--with-commit=<package>=<commit>'.

The '--with-source' flag also changes the source, but uses the syntax '--with-source=<url or file>', but only transforms the top-level package (e.g. `guix build <top-level package>` or `guix environment --ad-hoc `<top-level package>`). It also only works if the new source file has the form '<package name>-<package version>.<extension>', which means if a package source URL isn't named in that way, then you have to first download it, rename it to fit that template, then use --with-source with that file.

I propose changing the syntax of '--with-source=' to match the syntax of '--with-commit='. This would have the following benefits:

 - No need to download and rename files that don't fit the '<package name>-<package version>.<extension>' format.
 - Able to change the source file of a package input of any package in the package graph.
 - Makes the syntax consistent with '--with-commit='.

What do other people think?