### Description
This PR is a first step in a series of cleanups of action relationships. This first step does not contain any behavioral change, and it simply reorganizes / prunes / rearranges / documents the code. Mainly:
- it divides some files in RQL.Types between metadata types, schema cache types, execution types;
- it renames some types for consistency;
- it minimizes exports and prunes unnecessary types;
- it moves some types in places where they make more sense;
- it replaces uses of `DMap BackendTag` with `BackendMap`.
Most of the "movement" within files re-organizes declarations in a "top-down" fashion, by moving all TH splices to the end of the file, which avoids order or declarations mattering.
### Optional list types
One main type change this PR makes is a replacement of variant list types in `CustomTypes.hs`; we had `Maybe [a]`, or sometimes `Maybe (NonEmpty a)`. This PR harmonizes all of them to `[a]`, as most of the code would use them as such, by doing `fromMaybe []` or `maybe [] toList`.
PR-URL: https://github.com/hasura/graphql-engine-mono/pull/4613
GitOrigin-RevId: bc624e10df587eba862ff27a5e8021b32d0d78a2
## Description
This PR adds a config file for [`weeder`](https://github.com/ocharles/weeder) to the `-mono` repository. `weeder` checks for dead code by building a call graph from the given entry points (currently every module named `Main` with a `main` function) and then marking every function _not_ in that call graph as dead code.
To avoid very large PRs, I'm going to tackle this in a series. This first PR adds the basic configuration, plus removes as many weeds as it took for me to realise this was going to become a very big PR. The PRs after this will largely be removing dead code, until the final PR that will add Weeder to the CI pipeline.
### Related Issues
This closes#2973.
## Affected components
- Server
PR-URL: https://github.com/hasura/graphql-engine-mono/pull/4572
GitOrigin-RevId: ac8eaa9473e5ac1f16babcb35388694392d0d7dc
