exclude: in setup.ini

[Robert Collins]

Chris,
    this tag isn't documented in setup.html, can you give me a quick run
down on why it's needed?

For local installs where packages are missing, I'm currently just
deleting the package when it's found to have no valid mirrors. So the
EXCLUDE_BY_SETUP case is the only one I haven't duplicated - and I'm
wondering if that has ever been used? (And if it woudn't be easier to
just not put the package in setup.ini?)

Rob