A rule that sees a whole package
Replaces #7, which GitHub will not let me reopen: I merged it by mistake and
then took it back off master.
Stacked on #10. Review that first; this diff is against it.
Adds `HsGenValidInGenPackage` from `haskell-style.md`, the first rule that
cannot answer from one module, and with it the fact store that lets a rule see
further.
39 files, 1,198 lines. 263 tests.
## Why a database
What a rule above the module level does with facts is join them. One table per
fact, owned by the rule that writes it, so a rule brings its own schema the way
it brings its own check. The envelope never learns what is in that table, which
is why adding a rule adds no case to it: a package rule is a migration, what it
writes out of one module, and the query it answers with.
A fact is a row rather than a keyed entry. Two instances in one module are two
facts, and there is nothing to key them on that is not invented.
Where the store lives is a two-constructor type rather than a string, because
`:memory:` is not a file name: it is SQLite's word for having no file at all.
## The level
A rule's constructor is also its level, so nothing can disagree with it. Each
level judges exactly the suppressions naming its own rules, which is what keeps
unused-suppression detection sound without a global pass over the repository.
## Still not here
One process, so the store has nowhere to go and nothing crosses a process
boundary. Nothing reads what the compiler wrote down: the package check's
signature does not take the compiler's answers, because this rule has no use
for them.
weeder removed seven store queries on the way in that only a project rule would
want.
## Review loop
```
nix flake check
nix develop --command cabal test hopinion-test --test-options="--ai-executor"
```