Make several undocumented functions private

[kandersolar]

• Goes some way to addressing Functions to consider making private #1756
• I am familiar with the contributing guidelines
• Tests added
• Updates entries in docs/sphinx/source/reference for API changes.
• Adds description and name entries in the appropriate "what's new" file in docs/sphinx/source/whatsnew for all changes. Includes link to the GitHub Issue with :issue:`num` or this Pull Request with :pull:`num`. Includes contributor name and/or GitHub username (link with :ghuser:`user`).
• New code is fully documented. Includes numpydoc compliant docstrings, examples, and comments where necessary.
• Pull request is nearly complete and ready for detailed review.
• Maintainer: Appropriate GitHub Labels (including remote-data) and Milestone are assigned to the Pull Request and linked Issue.

This PR changes three things:

• Move pvlib.spa.calculate_deltat to pvlib.solarposition.calculate_deltat, since the calculation it implements is not part of the SPA
• Rename pvlib.spa to pvlib._spa to make the whole module private, since none of the remaining functions are intended to be used directly
• Rename the undocumented functions in Functions to consider making private #1756 to be private