Read the SQL aggregate a wrapper serves from its own tag

[estebanzimanyi]

An aggregate's transition, combine and final wrappers each back a CREATE FUNCTION nobody calls and implement a CREATE AGGREGATE everybody does, and @sqlfn holds one name. Where the aggregate name takes that slot the catalog states a name no CREATE FUNCTION carries, and a consumer separates an aggregate from a function by the C symbol's suffix rather than by what the source says. @sqlaggfn names the aggregate on the wrapper that serves it, beside the @sqlfn naming the function the wrapper backs. It rides the same @csqlfn chain as sqlfn and lands as sqlAggregate, a third fact distinct from both neighbours: sqlfn is the CREATE FUNCTION (tcount_transfn), sqlAgg the aggregate-role name MEOS states in @csqlaggfn (setUnionTransition), and sqlAggregate the CREATE AGGREGATE a binding registers (tCount). The reader is faithful — verbatim, no derivation — and is absent for a wrapper carrying no tag, so sources that state no @sqlaggfn yield the same catalog they yield today: generating from MobilityDB 6c107772f6 with and against master gives one sha256, and the run reports Attached 0 @sqlaggfn SQL aggregate names. tests/test_sqlaggfn.py covers the wrapper map, the chain, and the two tags coexisting on one block. The MobilityDB change that states the tag follows this one.