Revert original fix for BZ 57142 that attempted to:
- determine if the page might use EL
- if it did, create the ELContext (that would be required anyway) and set the
  necessary imports on the ImportHandler when the page loads

This alternative implementation:
- Generates a list of package and class imports at compilation time
- Which are then used when the ELContext is created

The main advantage of this approach is that the ELContext is only generated if it is definitely going to be used rather than if it might be used. There are also some minor optimisations regarding the timing of the String processing to differentiate between package and class imports.

git-svn-id: https://svn.apache.org/repos/asf/tomcat/trunk@1644516 13f79535-47bb-0310-9956-ffa450edef68