Do not install dependencies in build if lock file does not exist

[akalipetis]

We need to verify that the respective lock file exists (poetry.lock, pipenv.lock, requirements.txt, yarn.lock, etc), before adding the dependency installation command in the build steps.

          What if no `requirements.txt` is present?

Originally posted by @lolautruche in #26 (comment)

[lolautruche]

I'm not sure about actual lock files. My original comment aimed at the potential lack of requirements.txt file (equivalent to Pipfile or pyproject.toml)

[akalipetis]

We might end up in the exact same situation if the user selects Poetry, but there are no pyproject.toml and poetry.lock files in their repository, that's why I generalized it like that.

Same for package.json, yarn.lock, etc...

[vintorez]

What actions should we take if the user selects "pip" and we can't find the "requirements.txt"?
Should we keep the build steps empty?
Or perhaps, should we provide somewhere pip install -r requirements.txt as a suggestion/hint to the user?

[akalipetis]

This should be handled by #57 - we should remove the question altogether and automatically pick the correct dependency manager.