fix import completion, reuse JLine ClassCompletor, complete "as" fixed
keep as instance var so that other completors may benefit from search result
Also complete Classnames for preimported and imported classes

Example:
groovy:000> Sys<tab>

Before: no completion
Afer: completes to System  (from java.lang.System)

Also:
groovy:000> import java.awt.*
groovy:000> Sha<tab>

now completes to Shape