[Python-3000] Making 2to3 installable

["Martin v. Löwis"]

After talking to Collin, I just committed a change to
2to3 to make it installable as an executable script
called "2to3".

To do so, I had to wrap all the top-level modules
into a package, and move the fixes and tests into
that package. Lacking a better name, I called the
library lib2to3. I also install pgen2 as a separate
package.

In the process, I had to change all the imports,
and I did that by using relative imports. As
a consequence, 2to3 currently won't work on
Python 2.4.

Regards,
Martin