squash! Allow for getting NarInfo from dependency before packaging

Dependencies will be resolved as directories before they are packaged.
Therefore, we need to support extracting nar.properties from directories
in addition to .nar files.

While at it, refactor out a method to construct the path of the
nar.properties file, and let NarInfo's writeToFile() make the parent
directory when necessary.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Make writing nar.properties more convenient

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Write out nar.properties also after the 'compile' goal

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