Add --no-index flag to pip-compile

[atugushev]

Now we can finally bring back pip-compile --no-index in favour of consistency with pip-sync --no-index and pip install --no-index.

Background:

• --index/--no-index flag means different things in pip-compile and pip-sync #373
• Deprecate --index/--no-index in pip-compile #1130
• Remove deprecated "--index/--no-index" option from compile command #1234

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• Provided the tests for the changes.
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