[Distutils] distlib updated to include "entry point" functionality

[Vinay Sajip]

I've now updated distlib to include equivalent functionality for pkg_resources'
entry points. There is no ability to install packages on-demand via "require()"
functionality, but distributions are able to register things they implement,
and the capability to query what's available is also present. The relevant
information is stored in a dist-info file REGISTRY which has the same format as
entry_points.txt files. There is no code at present to write these files; that
will come later when focusing on what installer software will need from distlib.

The distlib.depgraph code has now been merged into distlib.database, as indicated
in an earlier post.

I've also added a section on migrating from older APIs. This currently covers
pkg_resources' resource and entry point APIs.

The resources API now covers extraction of files from .zip files and caching of
the extracted files in the file system.

I noticed that pkg_resources has a function cleanup_resources() to clear the
cache, but this is a no-op. Can someone tell me why this is?

Regards,

Vinay Sajip