Handle case where fastq_dir is a symlink

[nathanweeks]

Fixes issue #124 , checking if for a non-null/"file" scheme before attempting to resolve symlinks to avoid breaking cloud storage paths as supported in PR #122.

I manually tested (using --profile singularity) the case where a fastq_dir specified in the sample sheet is a symlink---not sure if it's worth adding an explicit test? Also, note that I'm only a "groovy novice", and suggestions containing a more robust or conventional implementation would be appreciated.

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!
• If you've added a new tool - have you followed the pipeline conventions in the contribution docs
• If necessary, also make a PR on the nf-core/spatialvi branch on the nf-core/test-datasets repository.
• Make sure your code lints (nf-core pipelines lint).
• Ensure the test suite passes (nextflow run . -profile test,docker --outdir <OUTDIR>).
• Check for unexpected warnings in debug mode (nextflow run . -profile debug,test,docker --outdir <OUTDIR>).
• Usage Documentation in docs/usage.md is updated.
• Output Documentation in docs/output.md is updated.
• CHANGELOG.md is updated.
• README.md is updated (including new tool citations and authors/contributors).

[fasterius]

Thanks a lot for this!