Instead it is provided to the user who can choose whether or not
to include it in the final derivati. Example of including would
be:
```nix
callPackage ... (self: { inherit (self.extras) extraThing; })
```
These extras are also available downstream without being built by
default. This is achieved with `passthru`.
- Only they are added to the optional build path (share/agda)
- Only they are are passed as an include dir (share/agda)
- Only they are propigatedBuildInputs
This is unused, future users can just use override `buildFlags`
and extend/replace as needed. `includeDirs` is provided for this
purpose.
We should add `dirOf self.everythingFile` rather than `.`, but
`dirOf` breaks on relative paths so that is not an option.
