fix `make_alias_definitions` for catlab compatibility

[kris-brown]

A small change: originally, for a binding with name → declaring an alias to Hom, we would create code to make an alias definition if the name → was not found in ext_functions (the explicitly imported things in the @instance declaration).

This was changed in a PR of mine because it seemed like one would more likely write import Hom rather than import →. To my surprise, this is causing regressions in Catlab. I haven't gotten to the bottom of why, but for the time being I'm rolling back this change.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (ed21d2e) 96.61% compared to head (1b89721) 96.61%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #133 +/- ## ======================================= Coverage 96.61% 96.61% ======================================= Files 38 38 Lines 2067 2067 ======================================= Hits 1997 1997 Misses 70 70 ```

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