Push secondary main tags into extratags

Push secondary main tags into extratags

When an object carries several main tags, Nominatim converts all of
them into categories but only places a single one into class/type.
The remaining main tags were dropped entirely, so they were no longer
accessible in the results (categories are not returned).

Keep secondary main tags in extratags instead of removing them from
the extratags set. Only the winning main tag that became class/type is
excluded from extratags. Because extratags are only collected when the
Lua style is configured to keep extra tags, the behaviour in the
default import (extra tags disabled) is unchanged.

Adds a BDD test covering secondary main tags being kept in extratags.