Now that imports can be limited to smaller scopes than the entire module,
unused import warnings need to work like unused def warnings.
This commit moves unused import warnings discovery and reporting from load
to canonicalization where we can track their usage per scope.
This also fixes a longstanding bug where unused exposed names from an import
were not reported if they were only used in a qualified manner.
Previously, all imports were available in the header, so we could start
processing dependencies as soon as we parsed it. However, the new imports
are treated as defs, so we have to parse the whole module to find them.
This commit essentially moves the dependency resolution from the `LoadHeader`
phase to the `Parse` phase, and it updates canonicalization to introduce
module symbols into scope when a `ValueDef::ModuleImport` is encountered.
NOTE:
- The `imports` header still parses, but it's no longer wired up. I will remove
it in an upcoming commit.
- Ingested files and imports that appear in nested expressions are not
yet supported by load
