c4ea0c8b

By: Tom Sydney Kerckhove <syd@cs-syd.eu>

Mutation: accept fully-qualified names in ignore and operator configs

The `ignore` filter previously matched only the bare OccName, and the
per-operator `skip-calls-to` configs matched only the bare name or the
name's defining module.  Neither let a user qualify a re-exported
function by the module they actually import it from
(e.g. `Data.Set.union`, defined in `Data.Set.Internal`).

Add `nameMatchCandidates`, built on the module's GlobalRdrEnv, which for
a name offers: the bare OccName (matching any module), the
defining-module qualification, and a qualification for every module the
name is imported through (from the GRE's import provenance).  Route the
ignore filter and RemoveAction's two ignore checks through a shared
`headNameMatches` helper, and have SwitchFunctionArguments and ElideCall
use the same candidate set.  Both helpers take the GlobalRdrEnv, threaded
from `instrumentEnvRdrEnv`.

Add `opName` (the Name-returning sibling of `opOccName`, now defined in
terms of it) to recover the head's module.

Exercise all three forms in nix/mutation.yaml and Example.IgnoreLib:
`mark` bare, `Example.IgnoreLib.markM` by defining module, and
`Control.Monad.Writer.censor` by import module (censor is re-exported
from Control.Monad.Writer.Class).  `censor` is used nowhere else, so the
global ignore entry cannot disturb other modules; the unchanged
IgnoreLib/ConstFnLib golden manifests are the regression test.

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