Run test as CI on salsa

With some minimal changes, the test suite for dh-python can be run on salsa, giving a green tick of approval to sit along side manual code review of changes.

The required changes are:

• decouple the running dh-python from its installation paths - there's only one location where pydist.py assumes things are in /usr/share/dh-python/dist/
• decouple the building of the test packages from an installed dh-python package - dpkg-buildpackage wants all the build-deps installed but that doesn't make sense for the dh-python ones

There are some additional changes in this MR to make the test output a bit more usable (mostly added to aid identifying and debugging the above issues!)

• add some visual separation between tests being run
• add some output showing the metadata of the packages that were created as part of each test
• add some additional debug logging to the dependency code in pydist to help understand where it was failing

Example salsa job and output: https://salsa.debian.org/stuart/dh-python/-/jobs/3262556

(There's also an unrelated but safe spelling typo test here that I can split off if that's easier)