Web fragments (attempt #2)

[Vlatombe]

• Reapply "[JENKINS-75174] Move existing web.xml to web-fragment.xml under core ([JENKINS-75174] Move existing web.xml to web-fragment.xml under core #10185)" (Revert "[JENKINS-75174] Move existing web.xml to web-fragment.xml under core (#10185)" #10225)
• Update jetty plugin configuration for configuration discovery

Tested with mvn -pl war jetty:run.

For running plugins with mvn hpi:run, jenkinsci/maven-hpi-plugin#710 is necessary and should be referred through the plugin parent pom before this gets merged.

See JENKINS-XXXXX.

Testing done

Proposed changelog entries

• human-readable text

Proposed upgrade guidelines

N/A

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1. The Jira issue, if it exists, is well-described.
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3. There is automated testing or an explanation as to why this change has no tests.
4. New public classes, fields, and methods are annotated with @Restricted or have @since TODO Javadocs, as appropriate.
5. New deprecations are annotated with @Deprecated(since = "TODO") or @Deprecated(forRemoval = true, since = "TODO"), if applicable.
6. New or substantially changed JavaScript is not defined inline and does not call eval to ease future introduction of Content Security Policy (CSP) directives (see documentation).
7. For dependency updates, there are links to external changelogs and, if possible, full differentials.
8. For new APIs and extension points, there is a link to at least one consumer.

Desired reviewers

@mention

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[Vlatombe]

While investigating some warnings during the startup, I realised that the jetty-maven-plugin already include core classes and resources from the module itself and not from the local maven repository (as long as mvn -pl war jetty:run is used).

Which means these extra classpath entries shouldn't be necessary to get live updates through hot reload.

Unfortunately, hot reload currently fails with

java.nio.file.ClosedFileSystemException
    at jdk.nio.zipfs.ZipFileSystem.ensureOpen (ZipFileSystem.java:1635)
    at jdk.nio.zipfs.ZipFileSystem.exists (ZipFileSystem.java:657)
    at jdk.nio.zipfs.ZipPath.exists (ZipPath.java:905)
    at jdk.nio.zipfs.ZipFileSystemProvider.exists (ZipFileSystemProvider.java:197)
    at java.nio.file.Files.exists (Files.java:2515)
    at org.eclipse.jetty.util.resource.PathResource.exists (PathResource.java:187)
    at org.eclipse.jetty.ee9.webapp.WebAppClassLoader.addClassPath (WebAppClassLoader.java:225)
    at org.eclipse.jetty.ee9.webapp.WebAppClassLoader.<init> (WebAppClassLoader.java:192)
    at org.eclipse.jetty.ee9.webapp.WebAppContext.configureClassLoader (WebAppContext.java:504)
    at org.eclipse.jetty.ee9.webapp.WebAppContext.preConfigure (WebAppContext.java:477)
    at org.eclipse.jetty.ee9.webapp.WebAppContext.doStart (WebAppContext.java:527)
    at org.eclipse.jetty.ee9.maven.plugin.MavenWebAppContext.doStart (MavenWebAppContext.java:313)
    at org.eclipse.jetty.util.component.AbstractLifeCycle.start (AbstractLifeCycle.java:93)
    at org.eclipse.jetty.ee9.maven.plugin.JettyEmbedder.redeployWebApp (JettyEmbedder.java:64)
    at org.eclipse.jetty.ee9.maven.plugin.JettyRunMojo.restartWebApp (JettyRunMojo.java:362)
    at org.eclipse.jetty.ee9.maven.plugin.JettyRunMojo$2.consoleEvent (JettyRunMojo.java:198)
    at org.eclipse.jetty.maven.ConsoleReader.signalEvent (ConsoleReader.java:64)
    at org.eclipse.jetty.maven.ConsoleReader.run (ConsoleReader.java:57)
    at java.lang.Thread.run (Thread.java:1583)

that seems to be caused by a JDK bug? (see jetty/jetty.project#11548)