Add core sources extras to requirements in `dlt init`

[burnash]

Implements #1794

Not sure if this is the best way to do it as there's a lot of implicit stuff and assumptions. Mostly this is to handle missing extra for the rest_api source.

The alternatives to reading package metadata are:

• explicitly define the core sources and their dependencies.
• have a requirements.txt as in verified sources and handle it the same way.

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[sh-rp]

@burnash from what I can tell this approach makes sense. What I would do: extend the dlt init tests to check the created requirements files and also do a manual test with each core source just to be sure it actually works.

[burnash]

What I would do: extend the dlt init tests to check the created requirements files and also do a manual test with each core source just to be sure it actually works.

Thanks @sh-rp, test are in 31bb59c