P2 metadata is generated each time in maven target locations

[laeubi]

The nature of maven target location is that it does not has P2 metadata at the first place. So we are currently generate it when we came along and resolve the target location and this includes:

1. The actual root item (a GAV)
2. Possibly dependent depending on settings.

Even though this is usually fast it is not fro free (especially for the second case). The goal is therefore that we first build up a key of options of the location that influence the generation of the metadata. Then we cache the metadata under this key alongside with the artifact unless the original artifact changes or updates are forced (similar to what is done with update sites already).