5136: Split namespace maps in `ItemScope` r=jonas-schievink a=jonas-schievink
Reduces memory usage of the CrateDefMap query by ~130 MB (50%) on r-a.
I was also looking into handling glob imports more efficiently (storing scope chains instead of always duplicating everything into the glob-importing module's scope), but it seems that this already gives the most significant wins.
Co-authored-by: Jonas Schievink <jonas.schievink@ferrous-systems.com>
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