Read directives with the defining compilation unit

The direct read of the imports, exports, and parts from the library
element is deprecated.

Unblocks https://dart-review.googlesource.com/c/sdk/+/388643

Author: natebosch

source_gen/lib/src/library.dart

@@ -37,12 +37,9 @@ class LibraryReader {
   Iterable<Element> get allElements => [
         element,
         ...element.topLevelElements,
-        // ignore: deprecated_member_use
-        ...element.libraryImports,
-        // ignore: deprecated_member_use
-        ...element.libraryExports,
-        // ignore: deprecated_member_use
-        ...element.parts,
+        ...element.definingCompilationUnit.libraryImports,
+        ...element.definingCompilationUnit.libraryExports,
+        ...element.definingCompilationUnit.parts,
       ];
 
   /// All of the declarations in this library annotated with [checker].