Regroup sentinels

[kdeldycke]

This is a small refactor to regroup all sentinels used across the Click codebase to the same private _utils.py file.

With this change:

• The new FLAG_NEEDS_VALUE sentinel is self-documenting. Compare:

  >>> from click.parser import _flag_needs_value
  >>> repr(_flag_needs_value)
  '<object object at 0x104798620>'

  with:

  >>> from click._utils import FLAG_NEEDS_VALUE
  >>> repr(FLAG_NEEDS_VALUE)
  'Sentinel.FLAG_NEEDS_VALUE'

• An option's _flag_needs_value property cannot be confused with the FLAG_NEEDS_VALUE sentinel as they are now clearly distinct by their naming convention.

[kdeldycke]

@Rowlando13 I guess this is a green-light from @davidism to merge this one! 😉

[kdeldycke]

Thanks @Rowlando13 for the merge! 😃