[SR-12075] Improve unknown package in dependencies of target diagnostic

Author: hartbit

Sources/PackageLoading/ManifestLoader.swift

@@ -979,7 +979,12 @@ extension TSCBasic.Diagnostic.Message {
     }
 
     static func unknownPackageInTargetDependencies(targetName: String, packageName: String) -> Self {
-        .error("unknown package '\(packageName)' in dependencies of target '\(targetName)'")
+        .error("""
+            unknown package '\(packageName)' in dependencies of target '\(targetName)'; if the package is named \
+            differently from the product, either use '.product(name: "\(packageName)", package: <package-name>)' to \
+            specify the package name or give the package the '\(packageName)' name using '.package(name: \
+            "\(packageName)", ...)'
+            """)
     }
 
     static func invalidBinaryLocation(targetName: String) -> Self {

Tests/PackageLoadingTests/PD5_2LoadingTests.swift

@@ -110,7 +110,11 @@ class PackageDescription5_2LoadingTests: PackageDescriptionLoadingTests {
                 """
 
             XCTAssertManifestLoadThrows(stream.bytes) { _, diagnostics in
-                diagnostics.check(diagnostic: "unknown package 'foo1' in dependencies of target 'foo'", behavior: .error)
+                diagnostics.check(diagnostic: """
+                    unknown package 'foo1' in dependencies of target 'foo'; if the package is named differently from \
+                    the product, either use '.product(name: "foo1", package: <package-name>)' to specify the package \
+                    name or give the package the 'foo1' name using '.package(name: "foo1", ...)'
+                    """, behavior: .error)
             }
         }
 
@@ -133,7 +137,11 @@ class PackageDescription5_2LoadingTests: PackageDescriptionLoadingTests {
                 """
 
             XCTAssertManifestLoadThrows(stream.bytes) { _, diagnostics in
-                diagnostics.check(diagnostic: "unknown package 'bar' in dependencies of target 'foo'", behavior: .error)
+                diagnostics.check(diagnostic: """
+                    unknown package 'bar' in dependencies of target 'foo'; if the package is named differently from \
+                    the product, either use '.product(name: "bar", package: <package-name>)' to specify the package \
+                    name or give the package the 'bar' name using '.package(name: "bar", ...)'
+                    """, behavior: .error)
             }
         }
     }