refactor!: Make MosaicTileset2D private

[kylebarron]

Change list

• Store the spatial index backing MosaicTileset2D in the MosaicLayer state instead of in the MosaicTileset state. This makes it easier to reason about because we can use standard layer lifecycle methods.
• Make MosaicTileset2D private. It should be an implementation detail only that MosaicLayer is backed by a deck.gl TileLayer
• Remove the unnecessary re-export of MinimalTileData type through the deck.gl-geotiff package