setup.py: do not support redefining install paths in sub-commands.

This is quite hard to support and not really helpful since 'install'
needs to provided for '--root' anyway. Instead, follow the common
distutils style of having all the paths in 'install' and inheriting them
in sub-commands.
setup.py: only set necessary install paths in sub-commands.

Remove the global mapping and instead just inherit the paths that we use
in the sub-commands.