* (allow to) bypass the file parsing if the pkgname is not wanted
* pass the WantPkgNames setting to pkgpaths and import_paths
* add support for `WantPkgNames`
* implement switching between the legacy `.a` imports and src imports
* add a setting to decide whether to use .a imports or search the pkg trees for packages
* implement the use_legacy_imports setting
* add a warning about `use_legacy_imports` effectively disabling `use_named_imports`