cabal-hooks: Demand rules that produce libraries

[alt-romes]

Describe the feature request
Currently, Cabal-hooks rules will only be demanded under the following condition, as specified in the RFC:

A rule is considered demanded if:

• it generates a Haskell module that is declared to be an autogenerated module of the component we are building, or
  another rule that is itself demanded depends on the output of the rule.

[…]

The issue is, as of now, both the specification and implementation will not demand rules that produce libraries (e.g. for extra bundled libs). Originally we were worried about the relative order in which other rules would be run relative to Haskell modules – but libraries don’t matter for those considerations because they will only get included in the last step (the linking invocation).

We should rectify the specification and implementation to accept rules producing libraries, something along the lines of a new bullet like:

• it generates a static or dynamic library that is declared to be an extra.(static|dynamic|bundled)-library of the component we are building, or
  another rule that is itself demanded depends on the output of the rule.

This came up in the context of a user having trouble with Cabal-hooks in Discourse: https://discourse.haskell.org/t/well-typed-blog-an-introduction-to-cabal-hooks-for-package-authors/11319/10?u=romes

[alt-romes]

Sam pointed out the change should happen in these lines and should be somewhat simple.

    -- Compute demanded rules.
    --
    -- SetupHooks TODO: maybe requiring all generated modules to appear
    -- in autogen-modules is excessive; we can look through all modules instead.
    autogenModPaths =
      map (\m -> moduleNameSymbolicPath m <.> "hs") $
        autogenModules $
          componentBuildInfo $
            targetComponent tgtInfo
    leafRule_maybe (rId, r) =
      if any ((r `ruleOutputsLocation`) . (Location compAutogenDir)) autogenModPaths
        then vertexFromRuleId rId
        else Nothing
    leafRules = mapMaybe leafRule_maybe $ Map.toList allRules
    demandedRuleVerts = Set.fromList $ concatMap (Graph.reachable ruleGraph) leafRules
    nonDemandedRuleVerts = Set.fromList (Graph.vertices ruleGraph) Set.\\ demandedRuleVerts