Use relative imports for intra-package imports

[Jason R. Coombs]

In [setuptools 848](https://github.com/pypa/setuptools/pull/848), a user is attempting to vendor pkginfo into setuptools as setuptools.extern.pkginfo, but the absolute imports prevent the relocated package from importing its own modules. Would you consider replacing these absolute imports with relative ones to ease the burden of relocating pkginfo?

[Tres Seaver (tseaver)]

Sorry I missed seeing this when it was submitted: somehow my mail notifications got switched off. I would be willing to merge such a change.

[Tres Seaver (tseaver)]

Note that there are tests which expect to be able to do 'import pkginfo', in order to then use the module for introspection. Relative import syntax doesn't support something like 'import .. as pkginfo', which would make it harder to do.

[Jason R. Coombs (jaraco)]

I'm trying to run the tests, but they always fail during the install of the sdist with:

```
Processing ./.tox/dist/pkginfo-1.4.1.zip
    Complete output from command python setup.py egg_info:
    Traceback (most recent call last):
      File "<string>", line 1, in <module>
      File "/private/var/folders/c6/v7hnmq453xb6p2dbz1gqc6rr0000gn/T/pip-6anodl25-build/setup.py", line 19, in <module>
        CHANGES = open(os.path.join(here, 'CHANGES.txt')).read()
    FileNotFoundError: [Errno 2] No such file or directory: '/private/var/folders/c6/v7hnmq453xb6p2dbz1gqc6rr0000gn/T/pip-6anodl25-build/CHANGES.txt'
```

This failure happens even in trunk. If I set `tox.skipsdist=True` in `tox.ini`, the tests then run. Is there a reason skipsdist isn't used? How do these tests work for others?

[Jason R. Coombs (jaraco)]

I'll open a separate ticket for that.

[Jason R. Coombs (jaraco)]

> Note that there are tests which expect to be able to do 'import pkginfo', in order to then use the module for introspection. Relative import syntax doesn't support something like 'import .. as pkginfo', which would make it harder to do.

In the PR, I've proposed only updating the core package with relative import support, and allow the tests to continue to assume the package is installed in the root. That minimal change is sufficient to satisfy the vendoring needs of a package like setuptools.

[Jason R. Coombs (jaraco)]

Thanks mate!

[Tres Seaver (tseaver)]

Fix released in pkginfo 1.4.2 (2018-03-14).