Re-structure the linkcheck builder

[AA-Turner]

cc: @jayaddison

This PR re-organises and re-structures the linkcheck builder.

Of note:

• All functions defined within functions are factored out into top-level functions or class methods
• Classes and methods have been re-arranged (Builder, PostTransform, Checker, Worker)
• TLS verification on sphinx.util.requests has been changed to not pass the Config object all the way down
• The Hyperlink object now stores the document path
• BuildEnvironment and Config objects are used to extract properties and are not stored as class attributes

A

[jayaddison]

@AA-Turner is the removal of the requests version from the Sphinx user-agent string an intentional part of this changeset?

Everything else looks good to me; I have one or two minor suggestions that I'll open as a separate pull request.