launcher: complain when duplicate DLLs are detected (#306)

## Link to GitHub Issue, if one exists
#304 

## Description of change
Complain loudly about DLL conflicts. Show warnings for each DLL that
exists in both modules directory and the root.

This is not based on LoadLibrary hooks, just enumerating DLLs via
filesystem access.

Deliberately not using `log_fatal` since these are not necessarily going
to crash the game. Some people just love to have messy installs and if
the game booted fine before, it should continue to boot.

## Testing
Tested with/without `-modules`, with/without duplicate DLLs.

src/spice2x/util/libutils.h

@@ -24,6 +24,8 @@ namespace libutils {
         return try_library(module_name.c_str());
     }
 
+    void check_duplicate_dlls();
+
     // get module handle helpers
     HMODULE get_module(const char *module_name);
     HMODULE try_module(const char *module_name);