In 3d3779b037 this got turned from a
`std::map<std::string, std::string>` to `std::map<IdString, IdString>`.
Consequently, this exposed some initialization sequencing issues (#1361).
Only initialize the map when it's first used, to avoid these static issues.
This fixes#1361.
Signed-off-by: Sean Cross <sean@xobs.io>