pkginfo fails when asking about itself

[htgoebel]

When running `pkginfo`pkginfo`, I get an error:
  No PKG-INFO found for package: pkginfo

Expected behaviour
============================

According to `pkginfo --help`, pkginfo is able to work with installed packages:
```
o an installed package: in this case, 'path' should be the importable name
  of the package.
```
I would expect it to do so, esp. whit itself.

How to reproduce
============================

$ pip3 install --user -U pip>=20.0
$ pip3 install --user pkginfo==1.5.0.1
$ pip3 show -f pkginfo
Name: pkginfo
Version: 1.5.0.1
…
Files:
  ../../../bin/pkginfo
  pkginfo-1.5.0.1.dist-info/INSTALLER
  pkginfo-1.5.0.1.dist-info/LICENSE.txt
  pkginfo-1.5.0.1.dist-info/METADATA
  pkginfo-1.5.0.1.dist-info/RECORD
  pkginfo-1.5.0.1.dist-info/WHEEL
  pkginfo-1.5.0.1.dist-info/entry_points.txt
  pkginfo-1.5.0.1.dist-info/top_level.txt
  pkginfo/__init__.py
…
$ pkginfo pkginfo
$ pkginfo pkginfo
~/.local/lib/python3.7/site-packages/pkginfo/installed.py:54: UserWarning: No PKG-INFO found for package: pkginfo
  warnings.warn('No PKG-INFO found for package: %s' % self.package_name)

[Tres Seaver (tseaver)]

Current setuptools installs the package metadata in a 'foo-x.y.z-dist-info' subudirectory of 'site-packages'.