Rename pip._internal.distributions.{source -> sdist}

Certain environments seem to be leaving behind empty folders in this
directory -- There was an older pip release that included a source/
folder here which isn't getting deleted for some reason. I have not
figured out the root cause why this happens yet. This is resulting in
ImportErrors since Python imports have a higher precedence for packages
compared to modules.

This commit changes the name we're trying to import from, which should
help prevent this mode of failure.

src/pip/_internal/distributions/__init__.py

@@ -1,4 +1,4 @@
-from pip._internal.distributions.source import SourceDistribution
+from pip._internal.distributions.sdist import SourceDistribution
 from pip._internal.distributions.wheel import WheelDistribution
 from pip._internal.utils.typing import MYPY_CHECK_RUNNING