deprecate validation that a resource localization exists for the default language (#4172)

motivation: the validation  / warning is redundant

changes:
* remove the validation for the default language
* adjust test
* refactor TargetSourcesBuilderTests so we can test for no diagnostics

rdar://86297221

Author: tomerd

Sources/PackageLoading/Diagnostics.swift

@@ -127,17 +127,6 @@ extension Basics.Diagnostic {
             localized variants will never be chosen
             """)
     }
-
-    static func missingDefaultLocalizationResource(
-        resource: String,
-        targetName: String,
-        defaultLocalization: String
-    ) -> Self {
-        .warning("""
-            resource '\(resource)' in target '\(targetName)' is missing the default localization \
-            '\(defaultLocalization)'; the default localization is used as a fallback when no other localization matches
-            """)
-    }
 }
 
 extension ObservabilityMetadata {

Sources/PackageLoading/TargetSourcesBuilder.swift

@@ -172,7 +172,6 @@ public struct TargetSourcesBuilder {
         diagnoseConflictingResources(in: resources)
         diagnoseCopyConflictsWithLocalizationDirectories(in: resources)
         diagnoseLocalizedAndUnlocalizedVariants(in: resources)
-        diagnoseMissingDevelopmentRegionResource(in: resources)
         diagnoseInfoPlistConflicts(in: resources)
         diagnoseInvalidResource(in: target.resources)
 
@@ -322,24 +321,6 @@ public struct TargetSourcesBuilder {
         }
     }
 
-    private func diagnoseMissingDevelopmentRegionResource(in resources: [Resource]) {
-        // We can't diagnose anything here without a default localization set.
-        guard let defaultLocalization = self.defaultLocalization else {
-            return
-        }
-
-        let localizedResources = resources.lazy.filter({ $0.localization != nil && $0.localization != "Base" })
-        let resourcesByBasename = Dictionary(grouping: localizedResources, by: { $0.path.basename })
-        for (basename, resources) in resourcesByBasename {
-            if !resources.contains(where: { $0.localization == defaultLocalization }) {
-                self.observabilityScope.emit(.missingDefaultLocalizationResource(
-                    resource: basename,
-                    targetName: target.name,
-                    defaultLocalization: defaultLocalization))
-            }
-        }
-    }
-
     private func diagnoseInfoPlistConflicts(in resources: [Resource]) {
         for resource in resources {
             if resource.destination == RelativePath("Info.plist") {