Only crash for mandatory fields used by dev mods (#60)

Dev mods -- mods which are being developed in the current development 
environment -- are located and provided by `MinecraftLocator`.

This allows mod dependencies in the dev environment to use the 
deprecated `mandatory` field (if they haven't been updated yet by
their respective authors) while still forcing the mod developer to
replace their own use of the field in their dev mods.

loader/src/main/java/net/neoforged/fml/loading/moddiscovery/ModInfo.java

@@ -217,12 +217,16 @@ public ModVersion(final IModInfo owner, final IConfigurable config) {
             this.modId = config.<String>getConfigElement("modId")
                     .orElseThrow(()->new InvalidModFileException("Missing required field modid in dependency", getOwningFile()));
             this.type = config.<String>getConfigElement("type")
+                    // TODO - 1.21: remove the fallback to the mandatory field
                     .map(str -> str.toUpperCase(Locale.ROOT)).map(DependencyType::valueOf).orElseGet(() -> {
                         final var mandatory = config.<Boolean>getConfigElement("mandatory");
                         if (mandatory.isPresent()) {
                             if (!FMLLoader.isProduction()) {
                                 LOGGER.error("Mod '{}' uses deprecated 'mandatory' field in the dependency declaration for '{}'. Use the 'type' field and 'required'/'optional' instead", owner.getModId(), modId);
-                                throw new InvalidModFileException("Deprecated 'mandatory' field is used in dependency", getOwningFile());
+                                // only error the mod being "developed" (i.e. found through the MOD_CLASSES) to prevent dependencies from causing the crash
+                                if (owner.getOwningFile().getFile().getProvider() instanceof MinecraftLocator) {
+                                    throw new InvalidModFileException("Deprecated 'mandatory' field is used in dependency", getOwningFile());
+                                }
                             }
 
                             return mandatory.get() ? DependencyType.REQUIRED : DependencyType.OPTIONAL;