Support migrations for the same DbContext in the same assembly for multiple providers

[manuelelucchi]

Currently if you have the same DbContext that needs multiple providers, you have to create a different assembly for each provider migrations (as stated here), this not only slows down the development but it forces the project architecture in a certain way (you need an assembly with the DbContext to be referenced by the multiple migrations assembly that then are referenced by the main project).
I think it would be better if the tools understand that if you create a migration with a different provider, it has to be treated as a different one, all in the same assembly (in different folders of course)

[ajcvickers]

@bricelam I looked into this a bit and I don't see an easy way to make it work and can't find an issue on the backlog for it.

[bricelam]

Currently, the path of least resistance is to use a separate DbContext type for each provider. (They can share a common base DbContext type.) This can be improved once we know which provider the migrations were generated for (also needed for issue #15174).