Package: adds _loaded, removes pm._currently_...
Package: adds _loaded, removes pm._currently_...

Adds a _loaded attribute on Package indicating if it has been loaded
(imported, from load()).

No longer set _currently_loading_package on the PackageManager when
importing from load() or dependencies().

hjanime/VisTrails,VisTrails/VisTrails,Nikea/VisTrails,celiafish/VisTrails,hjanime/VisTrails,minesense/VisTrails,VisTrails/VisTrails,minesense/VisTrails,minesense/VisTrails,VisTrails/VisTrails,celiafish/VisTrails,VisTrails/VisTrails,hjanime/VisTrails,Nikea/VisTrails,celiafish/VisTrails,minesense/VisTrails,celiafish/VisTrails,Nikea/VisTrails,VisTrails/VisTrails,hjanime/VisTrails,minesense/VisTrails,Nikea/VisTrails,hjanime/VisTrails