component: add SUBPACKAGES

This commit add SUBPACKAGES support to the framework.

Fix users of _LICENSE_DIR
Fix multi/dual license
Add annotation to packages about being a subpkg
Make do-depends aware of the subpackages
Add PKGBASE.subpkg variable
Fix actual-package-depends target
Fix dependencies in sub-packages
Implement LICENSE support
Fix PLIST_FILES.subpkg
Make DESCR.subpkg warning message shorter
Add SUBPACKES to the list of OPTIONS_HELPERS
Fix _strip_perms
Simplify METADIR as in the original patch
Fix _PLIST
Fix the case when there are no subpackages
Fix typo
Fix _PKGDIR initialization
Fix PLIST
Fix typo in RE
Fix METADIR
Fix type in PKGFILE
Fix [build|run]-depends-list targets
Initial import of the subpackage framework
This import is based on https://reviews.freebsd.org/D16457

PR: 275735
Differential Revision: https://reviews.freebsd.org/D40549