maven-helper: fall back to downloading .nar files when .jar was not found
maven-helper: fall back to downloading .nar files when .jar was
not found

When a project has the <packaging> tag 'nar', its extension becomes
.nar even if it is a regular .jar file.

Signed-off-by: Johannes Schindelin <53fb8db7833fca1e1746dd8592f61048d350f64c@gmx.de>

scijava/scijava-common