Automatically create modules directory

[krokicki]

Addresses #2563 to automatically create a ./modules dir when necessary for installing modules.

Combined with #2562, this allows easy installation of nf-core modules and subworkflows into non-nf-core pipelines, and reduces friction for reuse.

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

[github-actions]

This PR is against the master branch ❌

• Do not close this PR
• Click Edit and change the base to dev
• This CI test will remain failed until you push a new commit

---

Hi @krokicki,

It looks like this pull-request is has been made against the krokicki/nf-core-tools master branch.
The master branch on nf-core repositories should always contain code from the latest release.
Because of this, PRs to master are only allowed if they come from the krokicki/nf-core-tools dev branch.

You do not need to close this PR, you can change the target branch to dev by clicking the "Edit" button at the top of this page.
Note that even after this, the test will continue to show as failing until you push a new commit.

Thanks again for your contribution!

[krokicki]

Can't get the checks to run on this PR, I will reopen it.