Increase the isolation of environment variables by un-setting the unused

This way we can be assured our environment only contains the variables
defined by the toolset/xpkg.  This is helpful for keeping things like the
local PYTHONPATH from messing with this one.
xpkg list command now shows packages in sorted in order

It's nice to keep the order consistent.  We need a verbose mode that shows
package size and other stats as well.
WORK IN PROGRESS