Change the '--with-source' flag to use the syntax `--with-source=PACKAGE=SOURCE`

[pkill9 <pkill9@runbox.com>]

I propose changing the `--with-source` flag to use the syntax of
similar `--with-...` flags such as `--with-commit`, e.g.
`--with-source=PACKAGE=SOURCE`, because currently you need to
change a file to match the form "package-version.extension`, and it
searches the package graph to find the package to replace the input
with. It's annoying having to download a source file and change it's
filename just to have it work with this flag.

It would also be more consistent to match the syntax of the other
flags. To illustrate, this is the section of `guix build --help` that
shows the syntax of all these `--with-...` flags:

```
      --with-source=SOURCE
                         use SOURCE when building the corresponding package
      --with-input=PACKAGE=REPLACEMENT
                         replace dependency PACKAGE by REPLACEMENT
      --with-graft=PACKAGE=REPLACEMENT
                         graft REPLACEMENT on packages that refer to PACKAGE
      --with-branch=PACKAGE=BRANCH
                         build PACKAGE from the latest commit of BRANCH
      --with-commit=PACKAGE=COMMIT
                         build PACKAGE from COMMIT
      --with-git-url=PACKAGE=URL
                         build PACKAGE from the repository at URL
```