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refactor handling of multiple WIT paths and/or worlds (#213)

This primarily addresses an issue introduced in v0.22.0 such that targeting a
specific WIT path and world via the CLI failed if a Python dependency with a
`componentize-py.toml` file was found in the specified search path which also
had its own WIT path.  In that case, we'd return an error because we couldn't
find the CLI-specified world in the `componentize-py.toml`-specified WIT path,
without bothering to look in the CLI-specified path.  We had a test for a
similar scenario, but it wasn't covering this exact case.

Addressing the issue required significant refactoring of how multiple WIT paths
and/or worlds are juggled.  Hopefully the result is a bit easier to follow,
although it's still pretty hairy.