[NFC] Remove unused imports of TSCUtility.Diagnostics{Engine,} (#7019)

Since TSC is deprecated, we should avoid redundant imports for its modules and types.

Author: MaxDesiatov

Tests/PackageLoadingTests/PD_4_2_LoadingTests.swift

@@ -17,7 +17,6 @@ import PackageModel
 import SPMTestSupport
 import XCTest
 
-import class TSCBasic.DiagnosticsEngine
 import class TSCBasic.InMemoryFileSystem
 import enum TSCBasic.PathValidationError
 
@@ -776,9 +775,3 @@ class PackageDescription4_2LoadingTests: PackageDescriptionLoadingTests {
 #endif
     }
 }
-
-extension DiagnosticsEngine {
-    public var hasWarnings: Bool {
-        return diagnostics.contains(where: { $0.message.behavior == .warning })
-    }
-}

Tests/WorkspaceTests/WorkspaceTests.swift

@@ -26,7 +26,6 @@ import XCTest
 import struct TSCBasic.ByteString
 import class TSCBasic.InMemoryFileSystem
 
-import enum TSCUtility.Diagnostics
 import struct TSCUtility.Version
 
 final class WorkspaceTests: XCTestCase {