Repackaged the library with native (vs automatic) JPMS module support

[jhy]

Adds an explicit module-info.java file. As a native module, tools requiring modules such as jlink will operate correctly without warnings.

This makes jsoup a multi-release jar so it's still compatible with java 8. The module-info.java file is:

module org.jsoup {
    exports org.jsoup;
    exports org.jsoup.helper;
    exports org.jsoup.nodes;
    exports org.jsoup.parser;
    exports org.jsoup.safety;
    exports org.jsoup.select;

    requires transitive java.xml; // for org.w3c.dom out of W3CDom
}

Nullability annotations
I looked at having also a requires static jsr305 entry as well. That's a build time dependency for our javax nonnull annotations. However as that's an automodule, we get the compile warning Required filename-based automodules detected: [jsr305-3.0.2.jar]. Please don't publish this project to a public artifact repository!. So I'm keeping it out. Everything still works and downstream consumers will still get nullability warnings in their IDE. However jsoup developers will see edit-time errors / warnings on the import and use of these annotations. Perhaps now is a good time to implement #1992.

Java 8 tests
This change requires a minimum Java version of 9, and the min CI builder is changed to 11 (LTS). We should add a CI action to test a built jar under Java 8. Tracked in #2026.

Fixes #1943
Fixes #1466

[cowwoc]

Regarding the null annotations, most IDEs just look at the class name, ignoring the package name. I created local copies of these classes in my own library and it solved the problem.

Another option is to use moditect-maven-plugin to add module-info.java to libraries that are missing them and re-publish them under your groupId. Here is an example of what I did for the p6spy library:

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
	<modelVersion>4.0.0</modelVersion>
	<parent>
		<groupId>redacted</groupId>
		<artifactId>root</artifactId>
		<version>1.0-SNAPSHOT</version>
	</parent>
	<artifactId>add-module-info</artifactId>
	<description>Adds module-info.java to dependencies that are missing it</description>

	<build>
		<plugins>
			<plugin>
				<groupId>org.moditect</groupId>
				<artifactId>moditect-maven-plugin</artifactId>
				<executions>
					<execution>
						<id>add-modules</id>
						<phase>prepare-package</phase>
						<goals>
							<goal>add-module-info</goal>
						</goals>
						<configuration>
							<outputDirectory>${project.build.directory}/modules</outputDirectory>
							<modules>
								<module>
									<artifact>
										<groupId>p6spy</groupId>
										<artifactId>p6spy</artifactId>
										<version>${p6spy.version}</version>
									</artifact>
									<moduleInfoSource>
										module p6spy
										{
										requires java.sql;
										requires java.management;
										requires org.slf4j;

										uses com.p6spy.engine.logging.LoggingEventListener;
										uses com.p6spy.engine.spy.JdbcEventListenerFactory;
										uses com.p6spy.engine.event.JdbcEventListener;
										provides java.sql.Driver with com.p6spy.engine.spy.P6SpyDriver;

										exports com.p6spy.engine.spy;
										exports com.p6spy.engine.spy.option;
										exports com.p6spy.engine.logging;
										exports com.p6spy.engine.event;
										}
									</moduleInfoSource>
								</module>
							</modules>
							<overwriteExistingFiles>true</overwriteExistingFiles>
						</configuration>
					</execution>
				</executions>
			</plugin>
			<plugin>
				<groupId>org.apache.maven.plugins</groupId>
				<artifactId>maven-dependency-plugin</artifactId>
				<executions>
					<execution>
						<id>copy-sources</id>
						<phase>generate-sources</phase>
						<goals>
							<goal>copy</goal>
						</goals>
						<configuration>
							<artifactItems>
								<artifactItem>
									<groupId>p6spy</groupId>
									<artifactId>p6spy</artifactId>
									<version>${p6spy.version}</version>
									<classifier>sources</classifier>
								</artifactItem>
							</artifactItems>
							<outputDirectory>${project.build.directory}/sources</outputDirectory>
						</configuration>
					</execution>
				</executions>
			</plugin>
			<plugin>
				<groupId>org.apache.maven.plugins</groupId>
				<artifactId>maven-jar-plugin</artifactId>
				<configuration>
					<skipIfEmpty>true</skipIfEmpty>
				</configuration>
			</plugin>
			<plugin>
				<groupId>org.apache.maven.plugins</groupId>
				<artifactId>maven-install-plugin</artifactId>
				<executions>
					<execution>
						<id>default-install</id>
						<phase>install</phase>
						<configuration>
							<skip>true</skip>
						</configuration>
					</execution>
					<execution>
						<id>install-p6spy-with-modules</id>
						<phase>install</phase>
						<goals>
							<goal>install-file</goal>
						</goals>
						<configuration>
							<groupId>${project.groupId}</groupId>
							<artifactId>p6spy</artifactId>
							<version>${p6spy.version}</version>
							<packaging>jar</packaging>
							<file>${project.build.directory}/modules/p6spy-${p6spy.version}.jar</file>
							<sources>${project.build.directory}/sources/p6spy-${p6spy.version}-sources.jar</sources>
						</configuration>
					</execution>
				</executions>
			</plugin>
		</plugins>
	</build>
</project>

I then replaced all references from the original p6spy artifact to the new one. All the errors/warnings went away.

[jhy]

Thanks @cowwoc, that's helpful. I'm tracking the nullability assertion change in #2028. I recall that when we chose the javax package vs just local names it was to ensure Kotlin understood the assertions. But in practice I don't know that that does much.