`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 package builds were failing for two reasons:

 1. Python versions prior to 3.7 throw exceptions when attempting to print
    Unicode characters under a "C" locale. (#9076)

 2. We depended on `dh-systemd` which no longer exists in Debian Bullseye, but
    is necessary in Ubuntu Xenial. (#9073)

Setting `LANG="C.UTF-8"` in the build environment fixes the first issue.
See also: https://bugs.python.org/issue19846



The second issue is a bit trickier. The dh-systemd package was merged into
debhelper version 9.20160709 and a transitional package left in its wake.

The transitional dh-systemd package was removed in Debian Bullseye.

However, Ubuntu Xenial ships an older debhelper, and still needs dh-systemd.

Thus, builds were failing on Bullseye since we depended on a package which had
ceased existing, but we couldn't remove it from the debian/control file and our
build scripts because we still needed it for Ubuntu Xenial.

We can fix the debian/control issue by listing dh-systemd as an alternative to
the newer versions of debhelper. Since dh-systemd declares that it depends on
debhelper, Ubuntu Xenial will select its older dh-systemd which will in turn
pull in its older debhelper, resulting in no change from the status quo. All
other supported releases will satisfy the debhelper dependency constraint and
skip the dh-systemd alternative.

Build scripts were fixed by unconditionally attempting to install dh-systemd on
all releases and suppressing failures.

Once we drop support for Ubuntu Xenial, we can revert most of this commit and
rely on the version constraint on debhelper in debian/control.

Fixes #9076
Fixes #9073

Signed-off-by: Dan Callahan <danc@element.io>

This is basically a contrived way of adding a `Recommends` on `libpq5`, to fix #5653.

The way this is supposed to happen in debhelper is to run
`dh_shlibdeps`, which in turn runs `dpkg-shlibdeps`, which spits things out
into `debian/<package>.substvars` whence they can later be included by
`control`.

Previously, we had disabled `dh_shlibdeps`, mostly because `dpkg-shlibdeps`
gets confused about PIL's interdependent objects, but that's not really the
right thing to do and there is another way to work around that.

Since we don't always use postgres, we don't necessarily want a hard Depends on
libpq5, so I've actually ended up adding an explicit invocation of
`dpkg-shlibdeps` for `psycopg2`.

I've also updated the build-depends list for the package, which was missing a
couple of entries.

In the debian package, make the virtualenv symlink python to /usr/bin/python3.X
rather than /usr/bin/python3. Also make sure we depend on the right python3.x
package.

This might help a bit with subtle failures when people install a package from
the wrong distro (https://github.com/matrix-org/synapse/issues/4431).

Otherwise people can't upgrade from matrix-synapse without removing it first

...  to allow installation alongside our matrix-synapse transitional package.

since #4298, the optional dependencies are no longer installed with a simple
`pip install .`, which meant that they were not being included in the debian
package.

The easy fix to that is dh_virtualenv --extras, but that needs dh_virtualenv
1.1...