To help with the transition to type-graph based CodeGen, we need to
reduce dependencies on legacy CodeGen from other parts of OI.
Pieces of CodeGen referenced by other files, but which only return
drgn data have been moved into a new "drgn_utils" target, allowing
us to remove CodeGen dependencies for these functions.
The new "symbol_service" build target can be used in the future by
other files only needing access to symbol information and not full
CodeGen.
