Unify config section reader implementations

[jaraco]

In #292, I realized that the config parser replacement I'd developed in #285 repeats the work previously done in Distribution._read_sections. Let's harmonize and unify those two approaches.

[jaraco]

The main difference between the two implementations is that one parses the section values and the other does not. Another difference is the use of regex to parse the section headers. Probably either approach is viable, although not using regex would eliminate the concern raised in #292.