User opens a source file in a package and sets a
breakpoint. He then calls a method in the Interactions
Pane that hits the breakpoint, and the Interactions
Pane switches to a debug interpreter for the
breakpoint. Behind the scenes, DrJava places the new
debug interpreter into the source file's package and
imports all classes/packages which were imported at the
top of the source file.
The package part of this is easy, since we already have
a method which can parse out a document's package. We
don't have any logic to parse import statements yet, so
that might be a little more work.