pcp: Look for *.index to find archives instead of *.meta
pcp: Look for *.index to find archives instead of *.meta

When archives are compressed, their *.meta component will
become *.meta.xz, but the *.index component remains uncompressed.
Opening compressed archives works just fine, so with this change,
Cockpit is again able to show all the archives and not just the most
recent one.

Fixes #12365
Closes #12429

mvollmer/cockpit,moraleslazaro/cockpit,cockpit-project/cockpit,mvollmer/cockpit,garrett/cockpit,mvollmer/cockpit,cockpituous/cockpit,garrett/cockpit,deryni/cockpit,garrett/cockpit,deryni/cockpit,cockpituous/cockpit,martinpitt/cockpit,cockpit-project/cockpit,garrett/cockpit,cockpituous/cockpit,martinpitt/cockpit,cockpituous/cockpit,moraleslazaro/cockpit,martinpitt/cockpit,cockpit-project/cockpit,deryni/cockpit,deryni/cockpit,martinpitt/cockpit,cockpituous/cockpit,moraleslazaro/cockpit,cockpituous/cockpit,garrett/cockpit,cockpit-project/cockpit,martinpitt/cockpit,mvollmer/cockpit,moraleslazaro/cockpit,moraleslazaro/cockpit,deryni/cockpit,deryni/cockpit,cockpit-project/cockpit,mvollmer/cockpit,moraleslazaro/cockpit,deryni/cockpit,moraleslazaro/cockpit