Reword the scope definition again and reorder some sentences for clarity
    
    This is to address
    https://pagure.io/fesco/fesco-docs/pull-request/34#comment-128268:
    "Systems that implement access to third-party packaged software" was not clear, and this
    is replaced by "Tools that provide access to other software packaging ecosystems".
    
    The part "should work with FESCo" is replaced by "this requirement applies to the primary
    software management tools" and gnome-software is "downgraded" to be just an example.
    
    "Typically" is removed, a policy should not say "typically", because then it is not clear
    when it really applies.