RCP e4 EclipseLink Error: No Persistence provider for EntityManager named - eclipselink

Question

I am trying to apply a persistence framework to an eclipse rcp e4 application. For this purpose, I created an example of a feature project, almost following the tutorial https://www.vogella.com/tutorials/JavaPersistenceAPI/article.html, but adapting for rcp. However I got the error "Persistence provider for EntityManager named xxx not found". I have searched for a week but found no solution. Could someone please tell me, what the error really is and how to resolve it?
The persistence.xml is located in project tasks.model folder META-INF. The eclipse.jar and javax.persistence.jar are in another plugin project (Earlier I defined maven dependencies in pom.xml. Although eclipse reports no compilation errors, but running rcp product would result in an error that ClassDefNotFoundException for persistence, so putting javax.persistence.jar in a separate plugin removed this error). Some answers suggested that the persistence.xml should be in the folder src/resources/META-INF. But in a plugin project the folder META-INF is already in its default place. I even created an extra folder resources/META-INF and put the persistence.xml in that, but it did not help. If I created a normal maven project, then things work properly, also when I change the database to mysql in localhost. The errors appear when I convert the project into a plugin project for building rcp applications.
The error "No Persistence provider for EntityManager" occurs when I invoke the command:
factory = Persistence.createEntityManagerFactory("todos");
My persistence.xml file looks like:
<?xml version="1.0" encoding="UTF-8" ?>
<persistence xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://java.sun.com/xml/ns/persistence http://java.sun.com/xml/ns/persistence/persistence_2_0.xsd"
version="2.0" xmlns="http://java.sun.com/xml/ns/persistence">
<persistence-unit name="todos" transaction-type="RESOURCE_LOCAL">
<provider>org.eclipse.persistence.jpa.PersistenceProvider</provider>
<class>tasks.model.Todo</class>
<properties>
<property name="javax.persistence.jdbc.driver" value="org.apache.derby.jdbc.EmbeddedDriver" />
<property name="javax.persistence.jdbc.url" value="jdbc:derby:/tempGeo/simpleDB" />
<property name="javax.persistence.jdbc.user" value="test" />
<property name="javax.persistence.jdbc.password" value="test" />
<!-- EclipseLink should create the database schema automatically -->
<property name="eclipselink.ddl-generation" value="create-tables" />
<property name="eclipselink.ddl-generation.output-mode" value="database" />
</properties>
</persistence-unit>
</persistence>