When generating Entropy metadata out of Portage xpak information,
strip /* from SLOT (a.k.a. the sub-slots part).
At the same time, rework get_installed_package_metadata() to return
stripped SLOT information to upper layers.
Sub-slots have little meaning for Entropy packages, since rebuilds are
not its business and soname bumps are already properly detected at
dependencies calculation time. However, implementing this feature in
future might have sense, for some currently unknown reasons.
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