Use leaflet and react-leaflet as external packages

[pikhovkin]

See the beginning here

[pikhovkin]

I also prepare the ground for the dependencies between the Dash components to work. You could support my pull request.

[emilhe]

@pikhovkin Sorry for the long wait time, I have been caught up in other stuff. In the raw form, I had some issues with resource loading while testing the PR. I have merged the PR though, and I am now working on getting the resource loading working. When I get it working, I will push a new release.

[pikhovkin]

@emilhe thank you for the merge. Now it remains for these guys to approve this merge so that plugins work.

[emilhe]

@pikhovkin Did you do any testing of loading of async components after implementing your change? It seems after merging the PR, externally developed components work, but async loaded chunks that are part of dash-leaflet itself no longer works,