This implementation assumes that the author, author_email, maintainer,
and maintainer_email strings are comma-separated lists of authors or
maintainers respectively. A number of projects (including this one) use
that method to list multiple authors/maintainers (as per
e.g. https://stackoverflow.com/a/10005265), and for projects that list
only a single person in the field, this won't hurt. We can expand this
logic in the future to support other methods of specifying multiple
authors/maintainers if necessary, perhaps even to the point of adding
a command-line option to inform the plugin how to parse those fields.

These tests verify that WritePyproject._transform_contributors() does
something reasonable in every case I could think of.

These tests run various combinations of author and maintainer metadata
through the plugin. The tests only verify the "typical" cases; I'm
leaving it to the unit tests for _transform_contributors() to ensure
that weird cases are handled in the proper way.

I made a judgment call to treat any missing metadata as an empty string,
but to never generate an author or maintainer entry in pyproject.toml
unless at least one of the name or email address is nonempty. This was
an arbitrary choice, at least in the sense that I didn't do any research
to determine how those cases are supposed to be handled (if there is any
standard for them). If necessary we can fix up the behavior later.

I wrote these tests to use the project fixture rather than manually
constructing Distribution objects because if I try the latter way,
it looks like setuptools_scm takes over and computes the version number
from the actual setuptools-pyproject-migration project, rather than
using the hard-coded version 0.0.1, which causes the tests to fail.
Hopefully in a future commit we can fix that.

This fixes the test which was (intentionally) broken by adding support
for the author and maintainer fields. I noticed that test_everything()
actually did not include maintainer metadata before, so I added it -
just a copy of the authors.