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]

Codecov Report

All modified and coverable lines are covered by tests ✅

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

Additional details and impacted files

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

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[epatters]

Thanks Kris! Recommending to merge to fix the problem in Catlab, but if there is a lingering question here it may be worth creating an issue to track it.