Raise consistent ValueError for invalid EntryPoint.value

[jaraco]

Closes #488

• Refactored parsing and handling of EntryPoint.value.
• Raise a ValueError if no match.
• Also raise ValueError on construction if the value is invalid.
• Prefer a cached property, as the property is likely to be retrieved at least 3 times (on construction and for module:attr access).

[jaraco]

This change does introduce some performance degradation in the entry_points performance test:

===================================== perf =====================================
exercises.py:cached distribution: 0:00:00.000713 (+-1 day, 23:59:59.999988, -2%)
exercises.py:discovery: 0:00:00.000719 (+0:00:00.000002, 0%)
exercises.py:entry_points(): 0:00:00.010800 (+0:00:00.000700, 7%)
exercises.py:entrypoint_regexp_perf: 0:00:00.000070 (+-1 day, 23:59:59.999999, -1%)
exercises.py:uncached distribution: 0:00:00.001090 (+0:00:00, 0%)

+.7 ms (7%) in this particular run. I've seen other numbers like 3% on my local machine. I saw a larger increase without the cached property.

This seems to be an acceptable cost.