`autobuild install <package> --as-source <same package again>' errors out with "ERROR: no source url specified for freetype"

To reproduce

1. In the toplevel dir of a viewer source checkout, run

   autobuild install freetype --as-source freetype

Expected

Source code for the freetype viewer dependency is fetched, maybe by hg-cloning https://bitbucket.org/lindenlab/3p-freetype to a local directory

Observed

checking package freetype

installing freetype --as-source

ERROR: no source url specified for freetype

For more information: try re-running your command with --verbose or --debug