docs: Add descriptions for package_dir and packages find where

* package_dir is needed for both setup.cfg and setup.py
* The where to look for packages is given under options.packages.find
  for setup.cfg, and is passed as the "where" arg to setuptools.find_package
  for setup.py
* The terminology of "root package" comes from the old Distutils Examples
  docs (note that as of March 2021 these docs are still up as all the
  information in them has not yet been migrated to the setuptools docs)
  c.f. https://docs.python.org/3/distutils/examples.html#pure-python-distribution-by-package

Author: matthewfeickert

source/tutorials/packaging-projects.rst

@@ -188,11 +188,16 @@ an escape hatch when absolutely necessary.
 
     In the options category, we have controls for setuptools itself:
 
+    - ``package_dir`` is a collection of package names and directories.
+      An empty package name represents the "root package", so in this
+      case the root package is in the ``src`` directory.
     - ``packages`` is a list of all Python :term:`import packages <Import
       Package>` that should be included in the :term:`Distribution Package`.
       Instead of listing each package manually, we can use the ``find:`` directive
-      to automatically discover all packages and subpackages. In this case, the
-      list of packages will be ``example_pkg`` as that's the only package present.
+      to automatically discover all packages and subpackages and
+      ``options.packages.find`` to specify the ``package_dir`` to use. In this
+      case, the list of packages will be ``example_pkg`` as that's the only
+      package present.
     - ``python_requires`` gives the versions of Python supported by your
       project. Installers like pip will look back though older versions of
       packages until it finds one that has a matching Python version.
@@ -294,11 +299,15 @@ an escape hatch when absolutely necessary.
       which license your package is available under, and which operating systems
       your package will work on. For a complete list of classifiers, see
       https://pypi.org/classifiers/.
+    - ``package_dir`` is a dictionary with package names for keys and directories
+      for values. An empty package name represents the "root package", so in this
+      case the root package is in the ``src`` directory.
     - ``packages`` is a list of all Python :term:`import packages <Import
       Package>` that should be included in the :term:`Distribution Package`.
       Instead of listing each package manually, we can use :func:`find_packages`
-      to automatically discover all packages and subpackages. In this case, the
-      list of packages will be ``example_pkg`` as that's the only package present.
+      to automatically discover all packages and subpackages under ``package_dir``.
+      In this case, the list of packages will be ``example_pkg`` as that's the
+      only package present.
     - ``python_requires`` gives the versions of Python supported by your
       project. Installers like pip will look back though older versions of
       packages until it finds one that has a matching Python version.