When running ./package.sh to rebuild an already installed port, we would not
want to spend time re-downlodaing the same tarball again. Ideally, this should
use some sort of hash checking to ensure the file is not truncated or something,
but this is good enough for now.
Ports/.port_include.sh, Toolchain/BuildIt.sh, Toolchain/UseIt.sh
have been left largely untouched due to use of Bash-exclusive
functions and variables such as $BASH_SOURCE, pushd and popd.
Much redundancy is removed from package scripts with this system.
It also supports simple dependency management, uninstalling (through
BSD ports style plist files), cleaning up after itself (with clean,
clean_dist, clean_all commands), etc.
