Reorder ParameterSource

[madduck]

Please consider sorting ParameterSource in order of "explicitness", as this
would make it easier to test for how explicit a parameter is.

Currently, the values are:

1. COMMANDLINE
2. ENVIRONMENT
3. DEFAULT
4. DEFAULT_MAP
5. PROMPT

If I want to test whether a parameter has been explicitly provided, I need to
test whether that is not DEFAULT and not DEFAULT_MAP.

Consider this order instead:

1. PROMPT
2. COMMANDLINE
3. ENVIRONMENT
4. DEFAULT_MAP
5. DEFAULT

Now I can test x < DEFAULT_MAP. And if #2878 goes anywhere, CONFIGFILE could be slotted in before DEFAULT_MAP.

[kdeldycke]

I like the idea of having that natural order. Not necessarily to simplify the precedence code (although it might be a nice touch), but so the precedence policy of Click is diffused in the code itself.

As for extending this enum with CONFIGFILE I was thinking of doing the same for a long time in my own Click Extra project. But nowadays I think DEFAULT_MAP is good enough and that's the way to go: have configuration files setting their values to DEFAULT_MAP.

That's exactly what I am doing with Click Extra: https://github.com/kdeldycke/click-extra/blob/96719fdcd7916acb03ac2e13f0885aeb0c07d76c/click_extra/config.py#L1100-L1128

[kdeldycke]

@madduck I explored your idea in this PR: #3248 . Can you check it?