[bugfix] Use partition's and system's environment when autodetecting the topology

[ekouts]

In this PR we include the partition's and system's modules in the topology autodetecting script.

• For local partitions we temporarily change the environment of Reframe to include both system's and partition's modules and variables.
• For remote partitions we temporarily change the environment to include system's modules and variables and add the partition's environment in the autodetecting script.

For this PR we also added support for addition in the SequenceView and MappingView classes.

Closes #2115.

[codecov-commenter]

Codecov Report

Merging #2140 (5a3486b) into master (5b17b95) will decrease coverage by 0.00%.
The diff coverage is 68.18%.

@@            Coverage Diff             @@
##           master    #2140      +/-   ##
==========================================
- Coverage   86.30%   86.29%   -0.01%     
==========================================
  Files          53       53              
  Lines        9563     9571       +8     
==========================================
+ Hits         8253     8259       +6     
- Misses       1310     1312       +2     

Impacted Files	Coverage Δ
reframe/frontend/autodetect.py	61.87% <60.00%> (+0.96%)	⬆️
reframe/utility/__init__.py	92.75% <83.33%> (-0.13%)	⬇️
reframe/core/schedulers/slurm.py	52.43% <100.00%> (ø)

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[vkarak]

Can you also update the description of the PR? Now you load the system modules locally, which I think is the correct thing.