FR: Add type-safety when specifying `needs` of the jobs

[meskill]

For now dependencies between jobs are specified as strings without any validation that could lead to invalid workflow file on generation.

[meskill]

Reopening the issue since there are bugs from #70 in case user forgot to add dependency job with add_job the generated workflow is not valid:

• The job id generated wrongly
• The are duplicates of build job

[meskill]

The first issue is fixed by #94

[tusharmath]

@meskill I think this is fixed now in b6890cf

Can you verify this case again.

[meskill]

@meskill I think this is fixed now in b6890cf

Can you verify this case again.

Yes, looks like it fixed now.

I've added unit tests #112 to make sure