subrepo: add "_cachestorehashvfs" to handle cache store hash files via vfs This "vfs" object will be used by subsequent patches to handle cache store hash files without direct file APIs. This patch decorates "_cachestorehashvfs" with "@propertycache" to delay vfs creation, because it is used only for cooperation with other repositories. In this patch, "/" is used as the path separator, even though "self._repo.join" uses platform specific path separator (e.g. "\\" on Windows). But it is reasonable enough, because "store" and other management file handling already include such implementation, and they work well.