always use `_mount`, even if top-level

[Rich-Harris]

Using this._mount(target, anchor) instead of this._fragment.m(target, anchor) would fix sveltejs/svelte-loader#43. Using a private _mount method like this is arguably a bit of a hack, and we could discuss whether we need to make it a public method for the sake of things like this, but for now it's an easy way to get granular HMR working.