Move typesForMimeType helper from BasicIndexingJob to anonymous namespace

Summary:
Removes the KFileMetadata include dependency in the header.

While at it, remove the temporary QList<KFM::Type::Type>, just iterate
through the hash.

Depends on D17324

Reviewers: Baloo, Frameworks, astippich, ngraham

Reviewed By: astippich

Subscribers: kde-frameworks-devel

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D17338