Elixir compiler warns on unused and duplicated aliases

Elixir landed compiler warnings for unused and duplicated aliases, resolving a long-standing request to surface alias declarations that never affect the compiled code [1]. The change covers two distinct cases: aliases that go unused because another alias shadows them, and aliases declared more than once in the same scope [2].

The implementation reworks how the compiler tracks aliases internally, storing `%{alias => [{mod, meta1}, ...]}` instead of the previous `%{{alias, mod} => meta}`, so that each alias can carry per-module metadata [1]. That structure is what lets the compiler mark one entry as used and still flag the duplicate, since only one of the `{mod, meta}` tuples is changed to `{mod, :used}` [1].

For codebases that compile cleanly today, the new warnings can surface as noise on the next compile: shadowed and duplicated aliases that were previously silent now produce warnings under the same unused-alias machinery [1][2]. Because the tracking lives in the compilation environment map, the warnings apply wherever Elixir already reports unused aliases, and no new flags are introduced in the input.