[javadoc] IDEA-314508 Support external snippets in @snippet

Basic resolve and completion

GitOrigin-RevId: 3bb613e52e32516c0f4a6f6fa038bec29a312bbc

java/java-tests/testData/codeInsight/daemonCodeAnalyzer/javaDoc/resolve/pkg/SnippetRefs.java

@@ -0,0 +1,14 @@
+package pkg;
+
+/**
+ * {@snippet file="Test.java"}
+ * {@snippet class=Test}
+ * {@snippet class=<error descr="Snippet file is not found">Test1</error>}
+ * {@snippet class="Test"}
+ * {@snippet class='Test''}
+ * {@snippet class='sub.Test'}
+ * {@snippet class=<error descr="Snippet file is not found">'sub.Test2'</error>}
+ * {@snippet file="sub/test.txt"}
+ */
+public class SnippetRefs {
+}

java/java-tests/testData/codeInsight/daemonCodeAnalyzer/javaDoc/resolve/pkg/snippet-files/Test.java

@@ -0,0 +1,2 @@
+public class Test {
+}

java/java-tests/testData/codeInsight/daemonCodeAnalyzer/javaDoc/resolve/pkg/snippet-files/sub/Test.java

@@ -0,0 +1,4 @@
+package sub;
+
+public class Test {
+}

java/java-tests/testData/codeInsight/daemonCodeAnalyzer/javaDoc/resolve/pkg/snippet-files/sub/test.txt

@@ -0,0 +1 @@
+Empty