Fix packaging, I think.  Resolves #19.

- dywypi.plugins stopped being a namespace package, which didn't work
  anyway, and which prevented find_packages from finding it.

- Tests are no longer packaged.

- The dywypi_plugins namespace package placeholder is kinda useless and
  I don't want more junk files than necessary, so it's gone, and instead
  we just skip the error if it doesn't exist.

- Metadata was ancient wow.  bugs.veekun.com is pretty dang defunct.