Java: Fix external API name for nested types

This fixes the name of reported external APIs for nested types.
The `toString()` method of `getSourceDeclaration()` would report the
name of a type, but not the name of the enclosing type. This results
in missing information in the `UnsupportedExternalAPIs.ql` query.

For example, previously it would report:

```
org.zapodot.junit.db.Builder#build()
```

However, the `Builder` class does not exist in the package and is only
a nested type within `EmbeddedDatabaseRule`. The correct name should be:

```
org.zapodot.junit.db.EmbeddedDatabaseRule$Builder#build()
```

This name also matches the format of MaD.

java/ql/src/Telemetry/ExternalApi.qll

@@ -27,8 +27,8 @@ class ExternalApi extends Callable {
    */
   string getApiName() {
     result =
-      this.getDeclaringType().getPackage() + "." + this.getDeclaringType().getSourceDeclaration() +
-        "#" + this.getName() + paramsString(this)
+      this.getDeclaringType().getPackage() + "." + this.getDeclaringType().nestedName() + "#" +
+        this.getName() + paramsString(this)
   }
 
   private string getJarName() {