Move WorkflowRepo class into seperate file, since it will be shared b…

[MatthiasZepper]

Move WorkflowRepo class into separate file, since it will be shared with container handlers as well.

PR checklist

• [x] 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[bot]]

Codecov Report

:x: Patch coverage is 68.62745% with 48 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 77.02%. Comparing base (33830f9) to head (adc9129). :warning: Report is 647 commits behind head on dev.

Files with missing lines	Patch %	Lines
nf_core/pipelines/downloads/workflow_repo.py	68.66%	47 Missing :warning:
nf_core/__main__.py	50.00%	1 Missing :warning:

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[MatthiasZepper]

Superseded by #3634. Closing.