Specify patches for packages with local source

[Mekeor Melire <mekeor.melire@gmail.com>]

Is it possible to specify patches for packages with local source?

The `local-file` declaration doesn't allow a `patches` declaration.
Therefore, I'm guessing I'd have to use `origin`. But I'm not sure
whether either (method git-fetch) or (method url-fetch) allow to specify
a local path?

The manual states:

    URL-FETCH from ‘(guix download)’
         download a file from the HTTP, HTTPS, or FTP URL
         specified in the ‘uri’ field;

But is "file://" also supported? Can I write something as follows?

    (define-public own-st
        (package
            (inherit st)
            (source
                (origin
                    (method url-fetch)
                    (uri        "/path/to/st") ; like this ← ?
                   ;(uri "file:///path/to/st") ; or like this ← ?
                   (patches …)))))