Give partest its own classpath in build.xml.

Separate from (and larger than) pack.classpath. Then take
the opportunity to lower the comic/tragic level of duplication
in build.xml.
Give interactive tests their own target.

% ant test.interactive
Moved scaladoc code into src/scaladoc.

This leverages the preceding several commits to push scaladoc
specific code into src/scaladoc. It also renders some scanner
code more comprehensible.
Moved interactive code into src/interactive.

As with scaladoc, pushes presentation compiler specific
code into its separate source area.
Eliminated all forInteractive/forScaladoc uses.

This is the commit which brings it all together. The booleans
forInteractive and forScaladoc are now deprecated and are not
inspected for any purpose. All behavioral changes formerly
accomplished via tests of those flags are embodied in the globals
built specifically for those tasks.