[bugfix] Fix module backend implementation for Lmod versions < 8.2

[vkarak]

This PR introduces also a method for the module system backends that they can override in order to process the Python bindings output before executing the Python statements. We use this mechanism to fix the emitted Python commands from Lmod versions prior to 8.2, which are broken.

Fixes #1648.

[vkarak]

@mand35 Can you test if this PR works for your system?

[codecov-io]

Codecov Report

Merging #1654 (05957e4) into master (4bf3156) will decrease coverage by 0.06%.
The diff coverage is 18.18%.

@@            Coverage Diff             @@
##           master    #1654      +/-   ##
==========================================
- Coverage   87.72%   87.65%   -0.07%     
==========================================
  Files          45       45              
  Lines        7477     7485       +8     
==========================================
+ Hits         6559     6561       +2     
- Misses        918      924       +6     

Impacted Files	Coverage Δ
reframe/core/modules.py	58.89% <18.18%> (-0.57%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 4bf3156...05957e4. Read the comment docs.