Drop the JavaKeywords class (#1442)

Instead:
- Move its sole used method `#isValidIdentifier` to the `SourceCode`
  class.
- Delegate most of said method's implementation to
  `javax.lang.model.SourceVersion`.

error-prone-utils/src/main/java/tech/picnic/errorprone/utils/ConflictDetection.java

@@ -1,7 +1,5 @@
 package tech.picnic.errorprone.utils;
 
-import static tech.picnic.errorprone.utils.JavaKeywords.isValidIdentifier;
-
 import com.google.errorprone.VisitorState;
 import com.google.errorprone.util.ASTHelpers;
 import com.sun.source.tree.ImportTree;
@@ -47,7 +45,7 @@ public static Optional<String> findMethodRenameBlocker(
       return Optional.of(String.format("`%s` is already statically imported", newName));
     }
 
-    if (!isValidIdentifier(newName)) {
+    if (!SourceCode.isValidIdentifier(newName)) {
       return Optional.of(String.format("`%s` is not a valid identifier", newName));
     }
 

error-prone-utils/src/main/java/tech/picnic/errorprone/utils/SourceCode.java

@@ -17,6 +17,7 @@
 import com.sun.tools.javac.util.JCDiagnostic.DiagnosticPosition;
 import com.sun.tools.javac.util.Position;
 import java.util.Optional;
+import javax.lang.model.SourceVersion;
 
 /**
  * A collection of Error Prone utility methods for dealing with the source code representation of
@@ -28,6 +29,18 @@ public final class SourceCode {
 
   private SourceCode() {}
 
+  /**
+   * Tells whether the given string is a valid identifier in the Java language.
+   *
+   * @param str The string of interest.
+   * @return {@code true} if the given string is a valid identifier in the Java language.
+   * @see <a href="https://docs.oracle.com/javase/specs/jls/se17/html/jls-3.html#jls-3.8">JDK 17 JLS
+   *     section 3.8: Identifiers</a>
+   */
+  public static boolean isValidIdentifier(String str) {
+    return str.indexOf('.') < 0 && SourceVersion.isName(str);
+  }
+
   /**
    * Returns a string representation of the given {@link Tree}, preferring the original source code
    * (if available) over its prettified representation.

error-prone-utils/src/test/java/tech/picnic/errorprone/utils/SourceCodeTest.java

@@ -1,6 +1,8 @@
 package tech.picnic.errorprone.utils;
 
 import static com.google.errorprone.BugPattern.SeverityLevel.ERROR;
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.junit.jupiter.params.provider.Arguments.arguments;
 
 import com.google.errorprone.BugCheckerRefactoringTestHelper;
 import com.google.errorprone.BugCheckerRefactoringTestHelper.TestMode;
@@ -22,10 +24,41 @@
 import com.sun.source.tree.Tree;
 import com.sun.source.tree.VariableTree;
 import java.util.Optional;
+import java.util.stream.Stream;
 import javax.lang.model.element.Name;
 import org.junit.jupiter.api.Test;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.Arguments;
+import org.junit.jupiter.params.provider.MethodSource;
 
 final class SourceCodeTest {
+  private static Stream<Arguments> isValidIdentifierTestCases() {
+    /* { string, expected } */
+    return Stream.of(
+        arguments("", false),
+        arguments(".", false),
+        arguments("a.", false),
+        arguments(".a", false),
+        arguments("a.b", false),
+        arguments("public", false),
+        arguments("true", false),
+        arguments("false", false),
+        arguments("null", false),
+        arguments("0", false),
+        arguments("\0", false),
+        arguments("a%\0", false),
+        arguments("a", true),
+        arguments("a0", true),
+        arguments("_a0", true),
+        arguments("test", true));
+  }
+
+  @MethodSource("isValidIdentifierTestCases")
+  @ParameterizedTest
+  void isValidIdentifier(String string, boolean expected) {
+    assertThat(SourceCode.isValidIdentifier(string)).isEqualTo(expected);
+  }
+
   @Test
   void toStringConstantExpression() {
     BugCheckerRefactoringTestHelper.newInstance(