use magit-with-blob

Remove `magit-with-blob' instead of `packed-with-blob'.  Remove
the latter and the `packed-set-coding-system' it used.  Because
`magit-with-blob' is currently only defined in magit's next
branch define it if necessary.
distribute packed-git as a separate package

This is preferable because someone who just wants `packed.el' (most
likely as a dependency of `auto-compile') should not have to install
`magit' which is required by `packed-git.el'.