Preserve ShadowInfo when rewriting evidence
When the canonicaliser rewrites evidence of a Wanted, it
should preserve the ShadowInfo (ctev_nosh) field. That is,
a WDerive should rewrite to WDerive, and WOnly to WOnly.
Previously we were unconditionally making a WDeriv, thereby
rewriting WOnly to WDeriv. This bit Nick Frisby (issue Trac #16735)
in the context of his plugin, but we don't have a compact test
The fix is simple, but does involve a bit more plumbing,
to pass the old ShadowInfo around, to use when building
the new Wanted.