Fix Pyroma >= 2.4 initialization

[volans-]

As there have been some changes in the available checks in Pyroma
recently, make the initialization more robust and future-proof
dynamically creating the mapping of available checks and Prospector
codes, using PYRUNKNOWN for eventual new checks not yet mapped in
Prospector.

Closes #276

[coveralls]

Pull Request Test Coverage Report for Build 631

• 5 of 6 (83.33%) changed or added relevant lines in 1 file are covered.
• 295 unchanged lines in 19 files lost coverage.
• Overall coverage decreased (-4.5%) to 73.094%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
prospector/tools/pyroma/init.py	5	6	83.33%

Files with Coverage Reduction	New Missed Lines	%
prospector/formatters/text.py	1	64.29%
prospector/encoding.py	5	66.67%
prospector/finder.py	6	83.59%
prospector/tools/pylint/linter.py	7	64.71%
prospector/message.py	8	79.66%
prospector/tools/pylint/collector.py	10	86.21%
prospector/tools/pep257/init.py	10	80.65%
prospector/tools/pylint/indent_checker.py	11	80.77%
prospector/formatters/grouped.py	12	100.0%
prospector/config/init.py	12	80.65%

Totals
Change from base Build 630:	-4.5%
Covered Lines:	1448
Relevant Lines:	1981

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💛 - Coveralls