Feature request: create modules dir when it doesn't exist

[krokicki]

Description of feature

When installing modules into a non-nf-core pipeline, you have to create an empty modules directory manually, otherwise you get this rather unfriendly error:

ERROR    Can't find a ./modules directory. Is this a DSL2 pipeline?
╭─────────────────────────────── Traceback (most recent call last) ────────────────────────────────╮

...

│ dules/modules_json.py:70 in create                                                               │
│                                                                                                  │
│     67 │   │   new_modules_json = {"name": pipeline_name.strip("'"), "homePage": pipeline_url.s  │
│     68 │   │                                                                                     │
│     69 │   │   if not self.modules_dir.exists():                                                 │
│ ❱   70 │   │   │   raise UserWarning("Can't find a ./modules directory. Is this a DSL2 pipeline  │
│     71 │   │                                                                                     │
│     72 │   │   # Get repositories                                                                │
│     73 │   │   repos, _ = self.get_pipeline_module_repositories("modules", self.modules_dir)     │
╰──────────────────────────────────────────────────────────────────────────────────────────────────╯
UserWarning: Can't find a ./modules directory. Is this a DSL2 pipeline?

It would be nice if the directory was just created automatically. It would make it easier to install modules and subworkflows in non-nf-core contexts.