[aggregate.d] use selective imports for dsymbolsem

The goal is to remove all semantic imports from AST node files. This makes it obvious what still needs to be done.

compiler/src/dmd/aggregate.d

@@ -25,7 +25,7 @@ import dmd.declaration;
 import dmd.dscope;
 import dmd.dstruct;
 import dmd.dsymbol;
-import dmd.dsymbolsem;
+import dmd.dsymbolsem : dsymbolSemantic, determineFields, search;
 import dmd.dtemplate;
 import dmd.errors;
 import dmd.expression;