Add support for type aliases

Currently, `componentize-py` generates no code for type aliases; instead, it just uses the dealiased types directly. Instead, we should generate assignment expressions for each alias and use the alias in all the same places (e.g. function parameter types, etc.) that the original WIT files used them.

No files or tests are named. Start by locating componentize-py’s type-generation and type-reference handling, then trace how WIT aliases are currently dealiased in generated Python. Done means emitting an assignment for every alias and retaining that alias wherever the original WIT uses it, including function parameter types.

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