Remove EggPluginManager, EggBasketPluginManager, PackagePluginManager

[mdickinson]

The EggPluginManager, EggBasketPluginManager and PackagePluginManager classes were deprecated in #540 (targeting the 7.0 release). They should be removed (along with supporting tests and machinery) in Envisage 8.0.0.

[mdickinson]

Another reason for the deprecation and removal: pkg_resources is now deprecated