Rework @PropertySource early parsing logic

Rework the @PropertySource parsing logic recently changed in commit
7c608886 to deal with the same source appearing on a @Configuration
class and an @Import class.

Processing now occurs in a single sweep, with any previously added
sources being converted to a CompositePropertySource.

Issue: SPR-12115

spring-core/src/test/java/org/springframework/core/env/CompositePropertySourceTests.java

@@ -0,0 +1,46 @@
+/*
+ * Copyright 2002-2014 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.core.env;
+
+import java.util.Collections;
+
+import org.junit.Test;
+
+import static org.hamcrest.Matchers.*;
+import static org.junit.Assert.*;
+
+/**
+ * Tests for {@link CompositePropertySource}.
+ * @author Phillip Webb
+ */
+public class CompositePropertySourceTests {
+
+	@Test
+	public void addFirst() {
+		PropertySource<?> p1 = new MapPropertySource("p1", Collections.emptyMap());
+		PropertySource<?> p2 = new MapPropertySource("p2", Collections.emptyMap());
+		PropertySource<?> p3 = new MapPropertySource("p3", Collections.emptyMap());
+		CompositePropertySource composite = new CompositePropertySource("c");
+		composite.addPropertySource(p2);
+		composite.addPropertySource(p3);
+		composite.addPropertySource(p1);
+		composite.addFirstPropertySource(p1);
+		assertThat(composite.toString(), containsString("MapPropertySource [name='p1'], "
+				+ "MapPropertySource [name='p2'], MapPropertySource [name='p3']"));
+	}
+
+}