Fix a bunch of processing issues

[davidism]

This fixes the various processing issues I noticed while investigating #1649.

• When multiple=True is set and nargs > 1, default must be a list of tuples. Setting such a default is detected correctly to produce a type of Tuple.
• get_default is now tried in consume_value with the other sources, rather than being tried in full_process_value after calling process_value. This makes the processing more consistent. consume_value must now return a (value, ParameterSource) tuple.
• UUID and DateTime types were adjusted so that their convert method allows a value that is already the correct type. This was already the case for other types.
• Prompting is done in Option.consume_value instead of full_process_value to match the other sources.
• Add ParameterSource.PROMPT to track that the value was prompted for. This is currently set even if the user confirms the default value.
• Use the parameter source to determine if an argument was provided on the command line for completion, rather than checking if the value is None. This fixes an issue an argument with a default value wouldn't be completed. Also use the parameter source to check if a prompt should happen.
• Return None if a default was not provided and nargs > 1. multiple=True or nargs=-1 returns an empty tuple. The workaround described in Tuples as Multi Value Options throw an TypeError #472 of setting default=(None, None) still works, but is no longer necessary.
• Empty env vars are treated as if they weren't provided, rather than as an empty string (Path(exists=True, envvar='FOO') tries to interpret FOO="" as an existing path #1285). This was only applied to one branch in the env var processing code, it has been extended to the other cases.

• Fixes wrong type detected from default=tuple when multiple=True #1649
• Fixes Tuples as Multi Value Options throw an TypeError #472, closes Allow unspecified values for composite types #767
• Fixes Path(exists=True, envvar='FOO') tries to interpret FOO="" as an existing path #1285, extends 1285 fix path does not exist #1304

Checklist:

• Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
• Add or update relevant docs, in the docs folder and in code.
• Add an entry in CHANGES.rst summarizing the change and linking to the issue.
• Add .. versionchanged:: entries in any relevant code docs.
• Run pre-commit hooks and fix any issues.
• Run pytest and tox, no tests failed.