python: mkvenv: introduce TOML-like representation of dependencies

python: mkvenv: introduce TOML-like representation of dependencies

We would like to place all Python dependencies in the same file, so that
we can add more information without having long and complex command lines.
The plan is to have a TOML file with one entry per package, for example

  [avocado]
  avocado-framework = {
    accepted = "(>=88.1, <93.0)",
    installed = "88.1",
    canary = "avocado"
  }

Each TOML section will thus be a dictionary of dictionaries.  Modify
mkvenv.py's workhorse function, _do_ensure, to already operate on such
a data structure.  The "ensure" subcommand is modified to separate the
depspec into a name and a version part, and use the result (plus the
--diagnose argument) to build a dictionary for each command line argument.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>