QDoc: Parameterize QML lookup by search tree

QML node and type lookup currently search the whole forest. A caller
that needs to restrict a lookup to one imported module cannot reuse
that lookup without first selecting candidates from unrelated trees.

Accept an optional search tree in the QML lookup helpers and thread
it through module, type and member discovery. Existing callers omit
the parameter and retain their forest-wide lookup and fallback order.
No caller selects a tree yet, and generated output stays unchanged.

This prepares scoped explicit-link lookup without changing its
eligibility or the interpretation of existing references.

Task-number: QTBUG-137279
Pick-to: 6.12
Change-Id: I18ecb982df4b05d0b9fb89c3a1331347743845e3
Reviewed-by: Topi Reinio <topi.reinio@qt.io>