lister: Add new maven lister

The Maven lister retrieves the maven central indexes, exports them in a
convenient text format, and parse them to identify all src archives and
pom files in the maven repository. Then the pom files are downloaded and
analysed to find and yield any scm reference.

Note: This is a new version of the maven lister diff D6133 which takes
into account the initial round of reviews.

Related to T1724