Generate python3dist(toml) BuildRequires directly from the macro
    
    The macro checks if pyproject.toml exists and echoes the dependency early.
    
    For projects with pyproject.toml, this saves one installation round.
    Previously, the installation steps by %generate_buildrequires were:
    
     1. (python3-devel +) pip + packaging
     2. toml
     3. parsed dependencies from pyproject.toml
     4. ...
    
    Now they are:
    
     1. (python3-devel +) pip + packaging + toml
     2. parsed dependencies from pyproject.toml
     3. ...
    
    For projects without pyproject.toml, the number of rounds remains the same:
    
     1. (python3-devel +) pip + packaging
     2. setuptools + wheel
     3. ...
    
    This is also more consistent:
    The Python script now only outputs dependencies of the probed project,
    it no longer partially outputs dependencies for itself.