Add option to list active manifests

[pakk-minidose]

Hello,
with Julia 1.6.0 running the gc command now produces the following output:

(@v1.6) pkg> gc
      Active manifest files: 5 found
      Active artifact files: 80 found
      Active scratchspaces: 2 found

Julia 1.5.x also listed where are these manifests located. This was useful to track down and "invalidate" (e.g. rename) manifests in old projects, where local copy of packages does not have to be maintained (so that HDD space is released).

Would it be possible to return this feature? I did not find any other way to display the active manifests.

Thank you.

[fredrikekre]

#621