fix(ephemerals): don't cache empty ephemerals

[Aphosis]

Ephemerals were initially exposed to package commands as an ephemerals binding.

Since dcf77a2, they are also exposed to late-bound functions using the same strategy: reusing the existing list of resolved ephemerals.

The issue lies in the difference in timing between late-bound functions and package commands:

• Package commands are executed after the context have been resolved, so we get a properly resolved list of ephemerals
• Late-bound functions are executed during a solve, which means we don't have access to the resolved ephemerals yet

The previous implementation did not crash when accessing ephemerals in late-bound functions: instead, it defaulted to an empty list.

This behavior leads to a second issue: ephemerals were added and cached as part of the list of pre-resolve bindings.

This can lead to the following scenario:

• A package late-bound function accesses the ephemerals binding
• Since the context is not resolve yet, no resolved ephemerals is found and an empty list is returned and cached
• Later, in the package commands, we also access ephemerals
• The cached empty list is returned, meaning that most queries will likely fail

This commit addresses these issues the following way:

• Move ephemerals out of pre-resolve bindings for commands, so that they are always retrieved from the resolved context
• For late-bound functions, use resolved ephemerals only if a context is available

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: Aphosis / name: Aphosis (6707d24)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 61.30%. Comparing base (a4160c2) to head (6707d24).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2174   +/-   ##
=======================================
  Coverage   61.29%   61.30%           
=======================================
  Files         164      164           
  Lines       20568    20572    +4     
  Branches     3575     3575           
=======================================
+ Hits        12607    12611    +4     
  Misses       7089     7089           
  Partials      872      872           

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[Aphosis]

I didn't add any test yet because I'm unsure where it should be added in the test suite, and how the test package should be created.

A good candidate seemed to be in the TestContext class:

rez/src/rez/tests/test_context.py

Line 23 in aa10016

class TestContext(TestBase, TempdirMixin):

Also, the test requires a package with specific attributes, and I'm not sure if the package should be created inline in the test or stored somewhere under data/tests.