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this is a new object to handle multiple place for the same library (for
example if a library is bundled in a platform and installed in the
sketchbook).

- removed a lot of redundant code (especially in install/uninstall/list)
- some operations should be reimplemented using the generic functions
  in the `arduino/libraries` module.
- improved output of `lib list` command
- improved indexer

Fix #145

This field will keep the information coming from the package_index.json.

It would be better to keep the index and the current status in two
different trees, BTW this is a major refactoring and will be done in
the future.

This will be used in the library manager (right now the indexed
libraries are mixed with installed libraries)