postgis: cleanup
Another part of https://github.com/NixOS/nixpkgs/pull/38698, though I did cleanup even more.
Moving docs to separate output should save another 30MB.
I did pin poppler to 0.61 just to be sure GDAL doesn't break again next
time poppler changes internal APIs.
* postgresql: reorganize package and it's extensions
Extracts some useful parts of https://github.com/NixOS/nixpkgs/pull/38698,
in particular, it's vision that postgresql plugins should be namespaced.
