Allows rootcling to find it whatever rtti etc setting are.

Split IO part of metacling/rootclingTCling.cxx into utils/rootclingIO.cxx; the rest depends is meta not cling.

Cling should never see this; add comment about libCore/libCling use; constness; default ctor = default.

This file is needed by libCling (dictgen) *and* libCore (TClass/TSchemaRule).
Luckily it is inline-only, so we simply #include it from both.

Basic, ROOT-independent interfaces (stringview, TClassEdit) go to core/foundation.
metautils becomes clingutils; hide TClingUtils (former TMetaUtils) from include/.
TCling* move to core/metacling/src/.
I/O related code for rootcling moves to core/metacling/.
Other dictionary-generation related code moves to core/dictgen.

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- Doxygen format
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- Doxygen format
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