Prior to this commit, the policy and priority were hard-coded.
The default values are now the old hard-coded values.
This will allow to develop a cli to trigger forges listing with oneshot policy
and some priority tasks. Thus ingesting those faster and without manual
interventation as we currently do.
As discussed in T1389 to ingest all packages using base loader, it need
a list of all the tarballs for a pakage.
Hence modifified lister to recursively list all the tarballs for a
package with their last updated time.
Previously gnu lister was using same code as that of tarball loader
to download, unzip and read tree.json file.
To make the code consise the downloading method is changed to
requests library.
