Application libraries private, Distutils metadata available for console scripts and introspection

[Ben Finney <ben+debian@benfinney.id.au>]

Howdy all,

How can I specify to Pybuild that an application should have its modules
all in a private namespace, but have the Distutils metadata also
available to `pkg_resources` queries?

I install its libraries to an application-specific space with
`PYBUILD_INSTALL_ARGS = --install-lib=/usr/share/foo/`::

    $ find /usr/share/FooApp/
    /usr/share/FooApp/
    /usr/share/FooApp/lorem
    /usr/share/FooApp/lorem/__init__.py
    /usr/share/FooApp/lorem/dolor.py
    /usr/share/FooApp/lorem/sit.py
    /usr/share/FooApp/lorem/amet.py
    /usr/share/FooApp/FooApp-1.2.3.egg-info
    /usr/share/FooApp/FooApp-1.2.3.egg-info/PKG-INFO
    /usr/share/FooApp/FooApp-1.2.3.egg-info/requires.txt
    /usr/share/FooApp/FooApp-1.2.3.egg-info/entry_points.txt
    /usr/share/FooApp/FooApp-1.2.3.egg-info/not-zip-safe
    /usr/share/FooApp/FooApp-1.2.3.egg-info/dependency_links.txt
    /usr/share/FooApp/FooApp-1.2.3.egg-info/top_level.txt
    /usr/share/FooApp/ipsum
    /usr/share/FooApp/ipsum/__init__.py
    /usr/share/FooApp/ipsum/consecteur.py
    /usr/share/FooApp/ipsum/adipiscing.py
    /usr/share/FooApp/ipsum/elit.py

(The actual code base is a fork of ‘dput’ to modernise its packaging
<URL:https://notabug.org/bignose/dput/> and Python idioms.)

The application's Python package is managed like other operating system
files, but is not in the general Python namespace for other programs to
import. Good.

The application has “console scripts” defined in the Distutils
`entry_points` mapping:

    $ cat ./setup.py
    […]
        entry_points={
            'console_scripts': [
                "foo=FooApp.foo:main",
                ],
            },
    […]

which installs command-line programs at `/usr/bin/foo`, for example.
Good.

The distribution metadata (the `FooApp-1.2.3.egg-info` directory) is
also installed to the private directory though, which makes it invisible
to `pkg_resources`:

    $ /usr/bin/foo
    Traceback (most recent call last):
      File "/usr/bin/foo", line 5, in <module>
        from pkg_resources import load_entry_point
      File "/usr/lib/python2.7/dist-packages/pkg_resources/__init__.py", line 3084, in <module>
        @_call_aside
      File "/usr/lib/python2.7/dist-packages/pkg_resources/__init__.py", line 3070, in _call_aside
        f(*args, **kwargs)
      File "/usr/lib/python2.7/dist-packages/pkg_resources/__init__.py", line 3097, in _initialize_master_working_set
        working_set = WorkingSet._build_master()
      File "/usr/lib/python2.7/dist-packages/pkg_resources/__init__.py", line 651, in _build_master
        ws.require(__requires__)
      File "/usr/lib/python2.7/dist-packages/pkg_resources/__init__.py", line 952, in require
        needed = self.resolve(parse_requirements(requirements))
      File "/usr/lib/python2.7/dist-packages/pkg_resources/__init__.py", line 839, in resolve
        raise DistributionNotFound(req, requirers)
    pkg_resources.DistributionNotFound: The FooApp==1.2.3' distribution was not found and is required by the application

Other queries to `pkg_resources` for this application's distribution,
for example to get the distribution version or homepage URL, will also
fail.

How can I have Pybuild specify to Distutils that the application's
library modules should be installed away from the public namespace and
not available for general import, but also that the `pkg_resources`
functionality should find the distribution metadata where expected?

-- 
 \     “When I was a kid I used to pray every night for a new bicycle. |
  `\    Then I realised that the Lord doesn't work that way so I stole |
_o__)                   one and asked Him to forgive me.” —Emo Philips |
Ben Finney