python: allow it as a build-tool again (#49201)

Python was removed from being a build tool in #46980, due to issues
when reusing specs. This PR adds a new rule to match the interpreter
among different Python packages, in clingo.

It also adds a bunch of new "build-tools", so that specs like:
```
py-matplotlib backend=tkagg
```
can be concretized in one go.

Modifications:
- [x] Make `py-matplotlib backend=tkagg` concretizable
- [x] Add unit-tests to ensure situations like in #46980 do not happen

---------

Signed-off-by: Massimiliano Culpo <massimiliano.culpo@gmail.com>

etc/spack/defaults/concretizer.yaml

@@ -54,9 +54,15 @@ concretizer:
       # Regular packages
       cmake: 2
       gmake: 2
+      python: 2
+      python-venv: 2
       py-cython: 2
       py-flit-core: 2
+      py-pip: 2
       py-setuptools: 2
+      py-wheel: 2
+      xcb-proto: 2
+      # Compilers
       gcc: 2
       llvm: 2
   # Option to specify compatibility between operating systems for reuse of compilers and packages