This module scans potential modules used by perl programs, and returns
a hash reference; its keys are the module names as appears in %INC
(e.g. Test/More.pm); the values are hash references with this structure:

    {
        file    => '/usr/local/lib/perl5/5.8.0/Test/More.pm',
        key     => 'Test/More.pm',
        type    => 'module',    # or 'autoload', 'data', 'shared'
        used_by => [ 'Test/Simple.pm', ... ],
    }