[bugfix] Fix remote topology detection for pip installations

[vkarak]

What we do essentially is to fall back in a venv+pip install solution if we can't copy the standard source structure. In that case, we install reframe from pip in a venv and invoke reframe --detect-host-topology.

Also this PR removes the parallel launcher from the remote detection script as suggested in #2926.

Closes #2914.

FYI: @casparvl @jack-morrison.

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: -0.06% ⚠️

Comparison is base (dfa93c3) 87.35% compared to head (c8983c5) 87.29%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2978      +/-   ##
==========================================
- Coverage   87.35%   87.29%   -0.06%     
==========================================
  Files          60       60              
  Lines       11684    11692       +8     
==========================================
  Hits        10207    10207              
- Misses       1477     1485       +8     

Files Changed	Coverage Δ
reframe/frontend/autodetect.py	57.32% <0.00%> (-3.08%)	⬇️

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