We were still passing `ModuleIds` from `load` to `can`, but now
that imports can appear in any scope, we don't know which package
an unqualified module name belongs to from the top level.
We now pass `PackageModuleIds` instead and keep a Map of `ModuleName` to
`ModuleId` in `Scope`.
This also allow us to import multiple modules with the same name from different
packages as long as a unique alias is provided.
Allows a module to be imported with an alias:
import JsonDecode as JD
Import aliases must be unique and they cannot have the same name
as an imported module.
After parsing a module, we now recursively traverse the tree to find
all imports inside Defs, not just the top-level ones.
Previously, imported modules were available in the entire file,
but that's no longer the case. Therefore, Scope now keeps track of
imported modules and Env::qualified_lookup checks whether a module
is available in the provided scope.
Note: Unused import warnings are still global and need to be updated.
