Fix lookup of HyperSpec symbols
... BASE-URI: New argument errorp.
... LOOKUP: Don't use introspective-lookup for symbols that don't exist yet.
Otherwise INTERN can create useless symbols, which is most annoying in the
presence of package locks, where it errors out.
... INTROSPECTIVE-LOOKUP: Don't signal errors only because the package
doesn't support hyperdoc, because then CL symbols error out instead of
falling through to hyperspec-lookup.
... Use BASE-URI without errorp. Return NIL if no base uri is configured.
... FIXME: Perhaps we should default to an empty URI here and check for
presence of hyperdoc-lookup only?