[UnusedMethod] exempt @MethodSource with qualified method names

As mentioned in [the JUnit 5 docs](https://junit.org/junit5/docs/current/api/org.junit.jupiter.params/org/junit/jupiter/params/provider/MethodSource.html), the value within `@MethodSource` can be a fully-qualified name. This updates the code to handle this fact and adds some tests to try to flex it. The particular case we were hitting is the one with the `@Nested` test class using a parameter method in its parent.

Fixes #2411

COPYBARA_INTEGRATE_REVIEW=#2411 from HubSpot:qualified-method-source-upstream 19ce7fb
PiperOrigin-RevId: 381974583

core/src/main/java/com/google/errorprone/bugpatterns/UnusedMethod.java

@@ -254,14 +254,21 @@ private void handleMethodSource(MethodTree tree) {
             .findAny()
             // get the annotation value array as a set of Names
             .flatMap(a -> getAnnotationValue(a, "value"))
-            .map(y -> asStrings(y).map(state::getName).collect(toImmutableSet()))
-            // remove all potentially unused methods whose simple name is referenced by the
-            // @MethodSource
+            .map(
+                y -> asStrings(y).map(state::getName).map(Name::toString).collect(toImmutableSet()))
+            // remove all potentially unused methods referenced by the @MethodSource
             .ifPresent(
                 referencedNames ->
                     unusedMethods
                         .entrySet()
-                        .removeIf(e -> referencedNames.contains(e.getKey().getSimpleName())));
+                        .removeIf(
+                            e -> {
+                              Symbol unusedSym = e.getKey();
+                              String simpleName = unusedSym.getSimpleName().toString();
+                              return referencedNames.contains(simpleName)
+                                  || referencedNames.contains(
+                                      unusedSym.owner.getQualifiedName() + "#" + simpleName);
+                            }));
       }
     }
 

core/src/test/java/com/google/errorprone/bugpatterns/UnusedMethodTest.java

@@ -223,4 +223,56 @@ public void methodSource() {
             "}")
         .doTest();
   }
+
+  @Test
+  public void qualifiedMethodSource() {
+    helper
+        .addSourceLines(
+            "MethodSource.java",
+            "package org.junit.jupiter.params.provider;",
+            "public @interface MethodSource {",
+            "  String[] value();",
+            "}")
+        .addSourceLines(
+            "Test.java",
+            "import java.util.stream.Stream;",
+            "import org.junit.jupiter.params.provider.MethodSource;",
+            "class Test {",
+            "  @MethodSource(\"Test#parameters\")",
+            "  void test() {}",
+            "",
+            "",
+            "  private static Stream<String> parameters() {",
+            "    return Stream.of();",
+            "  }",
+            "}")
+        .doTest();
+  }
+
+  @Test
+  public void nestedQualifiedMethodSource() {
+    helper
+        .addSourceLines(
+            "MethodSource.java",
+            "package org.junit.jupiter.params.provider;",
+            "public @interface MethodSource {",
+            "  String[] value();",
+            "}")
+        .addSourceLines(
+            "Test.java",
+            "import java.util.stream.Stream;",
+            "import org.junit.jupiter.params.provider.MethodSource;",
+            "class Test {",
+            "  // @Nested",
+            "  public class NestedTest {",
+            "    @MethodSource(\"Test#parameters\")",
+            "    void test() {}",
+            "  }",
+            "",
+            "  private static Stream<String> parameters() {",
+            "    return Stream.of();",
+            "  }",
+            "}")
+        .doTest();
+  }
 }