Fix: Copy the API Viewer resources into the built plugin

As mentioned in JENKINS-66988, we currently are not producing the API
Viewer in the built plugin.

This is due to [0], in which the JPI plugin ignores any processing of
the WAR that we do separately.

As an intermediate solution, we can make sure that we copy the files
into the `src/main/webapp` directory, so it is present for the plugin.

This requires that we then exclude the files from Git, as we want them
to only be generated, but we need to make sure that `config.json` is
still included.

Finally, we make sure that the `jpi` task depends on the copying of
files, for standalone building of the plugin, and the `copyApiViewer`
depends on the build process for the `api-viewer` project.

[0]: jenkinsci/gradle-jpi-plugin#193

.gitignore

@@ -31,3 +31,6 @@ generated
 .bundle
 
 node_modules
+
+!job-dsl-plugin/src/main/webapp/api-viewer/config.json
+job-dsl-plugin/src/main/webapp/api-viewer

job-dsl-plugin/build.gradle

@@ -12,7 +12,6 @@ buildscript {
 
 plugins {
     id 'org.jenkins-ci.jpi' version '0.43.0'
-    id 'war'
 }
 
 java {
@@ -91,16 +90,22 @@ test {
     systemProperty 'jenkins.test.noSpaceInTmpDirs', 'true'
 }
 
-war {
-    into('api-viewer') {
-        from(project(':job-dsl-api-viewer').projectDir) {
-            include 'index.html'
-            include 'build/dist/**'
-        }
+task copyApiViewer {
+    copy {
+        from(project(':job-dsl-api-viewer').projectDir)
+        include 'index.html'
+        into 'src/main/webapp/api-viewer'
+    }
+    copy {
+        from(project(':job-dsl-api-viewer').buildDir)
+        include 'dist/**'
+        into 'src/main/webapp/api-viewer/build'
     }
-    dependsOn tasks.getByPath(':job-dsl-api-viewer:build')
 }
 
+copyApiViewer.dependsOn ':job-dsl-api-viewer:build'
+jpi.dependsOn 'copyApiViewer'
+
 dependencies {
     annotationProcessor 'net.java.sezpoz:sezpoz:1.13'
     annotationProcessor 'org.jenkins-ci:annotation-indexer:1.12'