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 <johannes.schindelin@gmx.de>
maven-helper: refactor commit_from_gav

... so it can be reused by other functions.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
maven-helper: add 'is-deployed /path/to/pom.xml' command

This command determines whether a project wants to be deployed or not. If it does want to, the exit code is 1, otherwise 0.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>