Deprecate ValidationResult::setSource

The last change added a huge explanation comment on how this method is
to be used. This commit now names the field `setSourceForAllViolations`
to make it more clear to the dependent code what the method does.

We should remove the `setSource` method together with the
CanValidateField trait.

Bug: https://phabricator.wikimedia.org/T383340

Author: gbirke

src/ValidationResult.php

@@ -30,6 +30,13 @@ public function getViolations(): array {
 		return $this->violations;
 	}
 
+	/**
+	 * @deprecated Use {@see setSourceForAllViolations()} instead
+	 */
+	public function setSource( string $sourceName ): self {
+		return $this->setSourceForAllViolations( $sourceName );
+	}
+
 	/**
 	 * Set source of an error in all violations.
 	 *
@@ -41,7 +48,7 @@ public function getViolations(): array {
 	 * that validate several sources. In those cases, the responsibility to
 	 * set the source is with the validator and not its calling code.
 	 */
-	public function setSource( string $sourceName ): self {
+	public function setSourceForAllViolations( string $sourceName ): self {
 		foreach ( $this->violations as $violation ) {
 			$violation->setSource( $sourceName );
 		}

tests/Unit/ValidationResultTest.php

@@ -28,12 +28,12 @@ public function testResultWithViolationsIsNotSuccessful(): void {
 		$this->assertSame( [ $violation1, $violation2 ], $result->getViolations() );
 	}
 
-	public function testSetSourceSetsSourceForAllViolations(): void {
+	public function testSetSourceForAllViolationsChangesAllViolations(): void {
 		$violation1 = new ConstraintViolation( 'value?', 'test_message_1' );
 		$violation2 = new ConstraintViolation( 'value!', 'test_message_2' );
 		$result = new ValidationResult( $violation1, $violation2 );
 
-		$result->setSource( 'a_test_field' );
+		$result->setSourceForAllViolations( 'a_test_field' );
 
 		$this->assertSame( 'a_test_field', $violation1->getSource() );
 		$this->assertSame( 'a_test_field', $violation2->getSource() );