seems like the URL is passed as-is to the sftp-client, which can't
handle the "sftp://" prefix:
amit0 myapp # ebuild --debug myapp-1.0.ebuild digest
>>> Downloading 'sftp://my.server.com:/home/public/mydistfile.tar.gz'
Connecting to sftp...
ssh: sftp: Name or service not known
Couldn't read packet: Connection reset by peer
No digest file available and download failed.