For runtime dependencies, install_requires should be used. For build
dependencies, a PEP 518-compliant `pyproject.toml` should be used.

Other dependencies can use extra requirements.

In the test case, dist.data_files needs to be sorted because the
current implementation loads the configuration files as a dictionary
with arbitrary order on Python < 3.6.

* fix #1419 PEP420: add find_namespace: directive

* fix #1419 PEP420: add find_namespace: directive to documentation

* fix #1419 PEP420: add tests

* fix #1419 PEP420: clean up code

* fix #1419 PEP420: fix typo in documentation

* fix #1419 PEP420: fix typo in documentation

* fix #1419 PEP420: clean up code

* fix #1419 PEP420: add changelog entry

* fixup! fix #1419 PEP420: add tests

* fix #1419 PEP420: cleanup code refactor markers

* #1420: Rename find_namespace_ns to find_namespace_packages

*  #1420: update changelog entry

Allow setting long_description_content_type externally

Some tools, such as PBR, might want to set
long_description_content_type during the parent object's
Distribution.__init__() call (during distutils setup_keywords entry
points).  However, that field is currently unconditionally overwritten
after these calls, erasing the value.

We would rather not duplicate the existing method of copying into
dist.metadata as done with project_urls.

This preserves the fields within Distribution.metadata described by
self._DISTUTIULS_UNUPPORTED_METADATA, or otherwise takes it from
arguments.

A test case that simulates setting the long description and overriding
the arguments is added.