modfetch: GetPkg use lookupListFromCache

[visualfc]

fix goplus/gop#1346

• Get: go install change go get
• GetPkg: use pkgpath by list, support github.com/my/pkg/v2/sub@ver mode

- check github.com/my/pkg/v2/sub@ver by list from GOMODCACHE
  github.com/my/pkg/v2/sub@ver
  github.com/my/pkg/v2@ver + check dir sub
  github.com/my/pkg@ver + check dir v2/sub

• support pkg@latest

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (10181f6) 100.00% compared to head (02d015f) 100.00%.
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #31   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            3         3           
  Lines          392       392           
=========================================
  Hits           392       392           

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