Subject: bug#42155: --with-source=PACKAGE=REPLACEMENT-SOURCE doesn't work recursively

From: pkill9 <[email-masked]>
Date: Wed, 1 Jul 2020 17:53:31 +0100

For example, `guix build --with-source=youtube-dl=blahblah mpv` builds
mpv normally, after giving a message: guix build: warning:
transformation 'with-source' had no effect on [email-masked]