Make sygus datatype building independent of parser in sygus v2 (#3923)

Make sygus datatype building independent of parser in sygus v2 (#3923)

The current sygus v2 called the parser's mkMututalDatatypeTypes function, which unecessarily created the datatype and bound its (internally generated) constructor/selector symbols in the symbol tables of the parser. This resolves this dependency.

The same issue also exists in the sygus v1 parser but is harder to resolve; I am leaving this for now since that code will be deleted in the next version of CVC4.

This is work towards the SyGuS API.