[bugfix] Fix in-class updates of inherited mutable variables

[vkarak]

The problem was that when variables from base classes where lowered in the current namespace as ShadowVars their value was not deep copied as in the past. As a result, when a mutable variable was updated in a subclass body, the original copy of the base class was being changed and this was visible to any new class inheriting it.

The solution is much more complicated as simply doing a deep copy of the original value, due to aliases. If we lower the root variable and simply deep copy its value, its alias still refers to the original one. In this case, for example, env_vars behaviour would have been fixed but not that of variables. As a result, whenever a variable is lowered, all of its aliases are also lowered. We essentially recreate the aliases as ShadowVars pointing to the ShadowVar of the lowered root variable.

Finally, a couple of tweaks were needed for deprecated aliases:

1. We need to replace the reference in the root variable to point to the deprecated version of the alias
2. We suppress any deprecation warnings whenever a deprecated alias is lowered.

Closes #2848.

[codecov]

Codecov Report

Patch coverage: 97.61% and project coverage change: +0.04 🎉

Comparison is base (f8245bc) 86.91% compared to head (2936403) 86.95%.

❗ Current head 2936403 differs from pull request most recent head 5cf262f. Consider uploading reports for the commit 5cf262f to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2880      +/-   ##
==========================================
+ Coverage   86.91%   86.95%   +0.04%     
==========================================
  Files          60       60              
  Lines       11509    11546      +37     
==========================================
+ Hits        10003    10040      +37     
  Misses       1506     1506              

Impacted Files	Coverage Δ
reframe/core/variables.py	95.68% <96.42%> (+0.26%)	⬆️
reframe/core/meta.py	99.13% <100.00%> (+0.03%)	⬆️

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