Remote dependency on distutils (#9125)

`distutils` is pretty much deprecated these days, and replaced with
`setuptools`. It's also annoying because it's you can't `pip install` it, and
it's hard to figure out which debian package we should depend on to make sure
it's there.

Since we only use it for a tiny function anyway, let's just vendor said
function into our codebase.

debian/control

@@ -31,7 +31,6 @@ Pre-Depends: dpkg (>= 1.16.1)
 Depends:
  adduser,
  debconf,
- python3-distutils|libpython3-stdlib (<< 3.6),
  ${misc:Depends},
  ${shlibs:Depends},
  ${synapse:pydepends},