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hi: Deterministic ImportedMods in Usages

Rodrigo Mesquita requested to merge wip/romes/hi-det into master 
hi: Deterministic ImportedMods in Usages

The `mi_usages` field of the interface files must use a deterministic
list of `Usage`s to guarantee a deterministic interface. However, this
list was, in its origins, constructed from a `ModuleEnv` which uses a
non-deterministic ordering that was leaking into the interface.

Specifically, ImportedMods = ModuleEnv ... would get converted to a list and
then passed to `mkUsageInfo` to construct the Usages.

The solution is simple. Back `ImportedMods` with a deterministic map.
`Map Module ...` is enough, since the Ord instance for `Module` already
uses a stable, deterministic, comparison.

Fixes

Fixes #25131 (closed)

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