It is neccessary for various purposes to get the
classpath for files related somehow to the
project. The java project is organized around the
CompilationUnits which are the primary source of
this classpath. However, usually the code starts
with some file and thus it would be usefull to
have some way how to get the classpath for the file.

The problem is that there are basically three
categories of files in the project:

1. properly added java source file (it belongs to
some CompilationUnit)
2. source file serving as a more rich information
source for some resource (e.g. library)
3. the java source which isn't marked as such
(e.g. it added only as a plain text file, it
doesn't belong to any CompilationUnit)

The helper can reliably work for the #1 only, for
the rest it can only try to do educated guess.