Move session loading logic into ghcide library (haskell/ghcide#697)

* Split out the session loading logic into a sublibrary

This way haskell-language-server can also reuse this logic.
Note that this sublibrary is public so it requires cabal-version: 3.0
Part of the work towards haskell/ghcide#478

* Move Development.IDE.Session into ghcide itself

Sublibraries do not seem to play well. Hide this behind the ghc-lib flag
so that the Haskell specific hie-bios stuff can be disabled

Note that we need to put the template-haskell part of this module into a
separate module because of an access exception when compiling with
Stack, GHC 8.10.1 and Windows.