refactor: move cache to pkg, simplify handlers

[avtakkar]

No description provided.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 69.72%. Comparing base (dd3559f) to head (44deeba).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #42      +/-   ##
==========================================
+ Coverage   69.37%   69.72%   +0.34%     
==========================================
  Files          34       34              
  Lines        1757     1757              
==========================================
+ Hits         1219     1225       +6     
+ Misses        538      532       -6     

Files	Coverage Δ
internal/files/store/mockstore.go	85.71% <ø> (ø)
internal/files/store/store.go	82.08% <100.00%> (ø)
internal/handlers/files/handler.go	84.61% <ø> (ø)
internal/handlers/root.go	48.88% <100.00%> (ø)
internal/handlers/v2/handler.go	39.13% <100.00%> (ø)
internal/handlers/v2/mirror.go	91.07% <ø> (ø)
internal/handlers/v2/registry.go	63.76% <ø> (ø)
pkg/cache/filecache.go	82.65% <100.00%> (ø)
pkg/cache/item.go	72.58% <ø> (ø)
pkg/cache/syncmap.go	93.54% <ø> (ø)
... and 1 more