localrepo: use file API via vfs while ensuring repository directory As a part of migration to vfs, this patch invokes some file API indirectly via vfs, while ensuring repository directory in the constructor of "localrepository" class. New file API are added to "scmutil.abstractopener" class, because they are also used via other derived classes than "scmutil.opener". But "join()" is not yet defined other than "scmutil.opener" class, because it should not be used via other opener classes yet.