when pipeline dir doesn't contain main.nf or nextflow.config

[mirpedrol]

Close #1941

Only warn when checking that the pipeline directory contains a main.nf and a nextflow.config file if the pipeline is not an nf-core pipeline

PR checklist

• This comment contains a description of changes (with reason)
• CHANGELOG.md is updated
• If you've fixed a bug or added code that should be tested, add tests!
• Documentation in docs is updated

[codecov]

Codecov Report

Merging #1964 (8eb75f1) into dev (263122a) will decrease coverage by 0.00%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##              dev    #1964      +/-   ##
==========================================
- Coverage   61.14%   61.13%   -0.01%     
==========================================
  Files          37       37              
  Lines        4676     4678       +2     
==========================================
+ Hits         2859     2860       +1     
- Misses       1817     1818       +1     

Impacted Files	Coverage Δ
nf_core/modules/modules_command.py	67.47% <66.66%> (-0.29%)	⬇️

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