Document __main__.__requires__

Also modernises the initial overview for pkg_resources, and helps
make it clear that it covers anything with an egg-info directory,
not just egg files.

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branch : docs_update_for_requires
Added tag 1.1 for changeset d943b67fe80d
Bumped to 1.2 in preparation for next release.
Update reference to distribute.
Add back a more descriptive intro

--HG--
branch : docs_update_for_requires
Typo fix

--HG--
branch : docs_update_for_requires
Merged in ncoghlan/setuptools/docs_update_for_requires (pull request #14)

Document __main__.__requires__
Merge update to ez_setup.py
Remove unused import
Remove unused import
Reorganize imports
Refactor for clarity
Move imports to leader
Refactor _dont_write_bytecode detection
More simply resolve _dont_write_bytecode.
Move imports to header
Normalize style
Refactor for nicer indentation
Rename variable to avoid conflation with global
Remove unused import