lib/*: Remove Exporter as "base" class

Lintian's use of "Exporter" is simply to obtain the "import" function.
Since Perl 5.8.3 (Exporter 5.57), this can be achieved by a simple:

  use Exporter qw(import);

In Lintian::Lab and Lintian::Lab::Entry, the export is completely
removed (only an "internal" constant were exportable).

(Included some unrelated "re-ordering" of use-statements for affected
modules.)

Signed-off-by: Niels Thykier <niels@thykier.net>