Fix get_plugins() bug

[BryceGattis]

Add suggested fix from @mmdanggg2 and add tests.

Closes #1618.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 58.43%. Comparing base (36e0537) to head (7b5b913).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1723      +/-   ##
==========================================
+ Coverage   58.29%   58.43%   +0.14%     
==========================================
  Files         126      126              
  Lines       17159    17160       +1     
  Branches     3505     3505              
==========================================
+ Hits        10002    10028      +26     
+ Misses       6493     6465      -28     
- Partials      664      667       +3     

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[JeanChristopheMorinPerso]

I'm wondering if we should instead make sure that get_latest_package returns valid packages. The docstring of get_latest_package says that it should either return a Package object or None is a package is not found. There is also this TODO

rez/src/rez/packages.py

Lines 925 to 926 in 71d990b

	# FIXME this isn't correct, since the pkg fam may exist but a pkg
	# in the range does not.

It my books, that's what I call smell.

[mmdanggg2]

In the case of #1618 I would say that get_latest_package is correctly returning None, as there is no valid package in an empty folder. Perhaps then iter_package_families should be more thorough and not return package names if there is no valid package in the folders?