ref: simplify flattenTree

[blaggacao]

People have been complaining about the readability of the code. In particular, the former flattenTreeSystem which might have ultimately lead to its abandonment instead of its fixing in the particular context it was abandoned.

Do a decisive preemptive strike, here.

[zimbatm]

I think we should get rid of flattenTree. It's not providing a lot of value, except using packages instead of legacyPackages.

[blaggacao]

I think the name-spacing by "/" can proove useful in itself and sets a good precedent. I have a sense it will be useful to export self.overlays with a flat keyspace that requires to include a composed key out of name + some other concept(s).

Though, the isDerivation check, as (now) assumed by filterPackages, maybe is in a dead spot, indeed, and could be removed cheaply.

[zimbatm]

I guess it doesn't hurt to keep it.

[zimbatm]

Closing as flattenTree is stable, and I don't want to touch it too much