Support retrieval of all EntryPoints in natural order

[jaraco]

The current entry_points API performs some processing on the entry points after gathering them from all of the distributions. As a result, there are two use-cases that are encumbered:

• It's inconvenient to retrieve the full set of entry points for the working environment. One must enumerate all groups and flatten the set of entry points to retrieve the full set in the environment.
• By the time the grouped entry points are returned, it's no longer possible to report or inspect the entry points in their natural order (discovery order), information that would prove useful in troubleshooting or for honoring environment-specific concerns, such as precedence.

The first use-case was reported to me by a custom packaging solution (like PyInstaller), though I haven't been able to find a reference.

It would be preferable to expose a low-level interface that presents the EntryPoints in their natural order before down-selecting them by group. Interestingly, I observe that both entrypoints and pkg_resources both allow resolution of entry points by group only (never the full set), suggesting that these use-cases are of low importance.

[jaraco]

Fixed in #278 (when 'select' is used, and later when SelectableGroups is dropped).