We take the idris2 projects version of the derivation. Originally,
Idris2 did not maintain their own nix derivation, so we created our
own. Now they maintain their own derivation, so we should try to
keep ours as close to theirs.
This change comes with the following differences:
* support files are in its own output, instead of packaged with idris2
- This makes it necessary to provide --package for contrib and network
!!! This is a breaking change !!!
* IDIRS2_PREFIX is set to ~/.idris2 instead of pointing to nix-store
- This makes --install work as expected for the user
* Properly set IDRIS2_PACKAGE_PATH
* non-linux platform uses chez-racket instead of chez
