Improvements for declarative configuration

[idlesign]

Hi,

1. read_configuration() now accepts ignore_option_errors argument. This allows scraping tools to read metadata without a need to download entire packages. E.g. we can gather some stats right from GitHub repos just by downloading setup.cfg.

2. packages find: directive now supports fine tuning from a subsection. The same arguments as for find() are accepted.

[idlesign]

@jaraco Fails on unrelated:

test_mixed_site_and_non_site
test_links_priority
test_setup_requires_honors_fetch_params

upd Sorry, there's some relation. Investigating.

[idlesign]

test_setup_requires_honors_fetch_params fail is fixed.

Yet seem to fail in master somehow:
test_mixed_site_and_non_site
test_links_priority

[idlesign]

@jaraco Fixed. This one was specific for test environment.

[jaraco]

I've rebased your commits onto the rolled-back master, now terminating at c471788.