Make segemehl outputs more specific

[nictru]

Currently, the output files of this module are all provided as a single channel containing the output directory.
This makes selecting individual files for further analysis inconvenient.

This was not a problem before, as the only nf-core pipeline using this module (circrna) had a specific process for handling this. Recently, we switched to using the nf-core GAWK module, which makes it necessary to provide only the relevant file to the process.

PR checklist

• This comment contains a description of changes (with reason).
• If you've fixed a bug or added code that should be tested, add tests!
• Follow the input/output options guidelines.
• Ensure that the test works with either Docker / Singularity. Conda CI tests can be quite flaky:
  • For modules:
    • nf-core modules test <MODULE> --profile docker
    • nf-core modules test <MODULE> --profile singularity
    • nf-core modules test <MODULE> --profile conda