Fix `cordova prepare` now that plugin assets are copied on (un)install

This change introduces a new platform/foo/.staging/www directory, into
which plugin assets are installed by plugman. cordova prepare copies
these on top of the platform-specific www directory.

The platform-specific www directories are still deleted and recreated on
every prepare.

Uninstallation is working, plugman tests are passing, CLI tests are
broken but this change isn't the problem.