URI::_punycode - Privatize functions

[genio]

Update URI::_punycode to make several functions that have never been documented and have always been considered private to actually be private.

• adapt -> _adapt
• code_point -> _code_point
• digit_value -> _digit_value
• min -> _min

This allows us to remove some special casing in the dist.ini conf file, reducing complexity.

[karenetheridge]

let's confirm via grep.cpan.me that this doesn't impact any cpan modules...

[oalders]

@genio were you able to grep CPAN for any issues?

[genio]

I'm not quite sure anymore. I kind of remember spending a few minutes thinking of how to search for any use of these functions using grep.cpan.me, but I didn't come up with anything and moved on.

[simbabque]

@haarg did a grep just now. Nothing uses URI::_punycode as far as we can see. Should be fine to merge.

[simbabque]

Thank you @genio.