Metadata simplify - #1883

setuptools natively supports declarative setup.cfg for a long time now, so there's no need to
specify this in pbr when it can be done in a more stock manner

Use of pbr at runtime is super ugly, as all it does is use pkg_resources to
parse distribution versions and pkg_resources could do the exact same thing. But
they are both slow, because pkg_resources is (by design) slow.

importlib.metadata was added to the stdlib in python 3.8 to fulfill this use
case in a speedy and elegant manner.

FIXME: this entire schema is ugly, not merely at runtime -- use of pbr for
versioning from git is an outdated and stale idea, this should use
setuptools_scm instead which:
- is standard
- supports writing the version to _version.py

Do not rely on running pip to install dependencies, which e.g. distros cannot
do. Instead let this be specified directly as package metadata.

While we are at it, stop actually running pip install if DESTDIR is set, since
this is a fairly likely sign that guake is being installed in a distro
environment where pip install is not allowed.