[refactor] Improve namespace injection

[jjotero]

Closes #2217

[codecov-commenter]

Codecov Report

Merging #2255 (d42b5a1) into master (37545d2) will decrease coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2255      +/-   ##
==========================================
- Coverage   86.13%   86.09%   -0.04%     
==========================================
  Files          56       56              
  Lines       10199    10172      -27     
==========================================
- Hits         8785     8758      -27     
  Misses       1414     1414              

Impacted Files	Coverage Δ
reframe/core/fixtures.py	100.00% <100.00%> (ø)
reframe/core/meta.py	98.79% <100.00%> (ø)
reframe/core/namespaces.py	96.07% <100.00%> (-0.65%)	⬇️
reframe/core/parameters.py	97.82% <100.00%> (-0.14%)	⬇️
reframe/core/variables.py	96.03% <100.00%> (-0.08%)	⬇️
reframe/frontend/cli.py	76.30% <0.00%> (+0.04%)	⬆️

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