Only allow directories, jars and zips on the lcp.

Closes: #144

userdev/src/functionalTest/groovy/net/neoforged/gradle/userdev/RunTests.groovy

@@ -48,6 +48,57 @@ class RunTests extends BuilderBasedTestSpecification {
         run.output.contains('runClient')
     }
 
+    def "when referencing pom on run classpath, it should not list it on the LCP, but it should list its dependencies"() {
+        given:
+        def project = create("runs_support_poms", {
+            it.build("""
+            java {
+                toolchain {
+                    languageVersion = JavaLanguageVersion.of(21)
+                }
+            }
+            
+            repositories {
+                mavenCentral()
+            }
+            
+            dependencies {
+                implementation 'net.neoforged:neoforge:+'
+            }
+            
+            runs {
+                client {
+                    dependencies {
+                        runtime 'org.graalvm.polyglot:python:23.1.2'
+                    }
+                    
+                    modSource project.sourceSets.main
+                }
+            }
+            """)
+            it.withToolchains()
+        })
+
+        when:
+        def run = project.run {
+            it.tasks(':writeMinecraftClasspathClient')
+        }
+
+        then:
+        run.task(':writeMinecraftClasspathClient').outcome == TaskOutcome.SUCCESS
+
+        def neoformDir = run.file(".gradle/configuration/neoForm")
+        def versionedNeoformDir = neoformDir.listFiles()[0]
+        def stepsDir = new File(versionedNeoformDir, "steps")
+        def stepDir = new File(stepsDir, "writeMinecraftClasspathClient")
+        def classpathFile = new File(stepDir, "classpath.txt")
+
+        classpathFile.exists()
+
+        classpathFile.text.contains("org.graalvm.polyglot/polyglot")
+        !classpathFile.text.contains(".pom")
+    }
+
     def "userdev supports custom run dependencies"() {
         given:
         def project = create("run_with_custom_dependencies", {

userdev/src/main/java/net/neoforged/gradle/userdev/runtime/tasks/ClasspathSerializer.java

@@ -25,7 +25,15 @@ public void run() throws Exception {
         final File out = ensureFileWorkspaceReady(getOutput());
         Files.write(
                 out.toPath(),
-                getInputFiles().getFiles().stream()
+                getInputFiles()
+                        .getAsFileTree()
+                        //Filter out valid classpath elements, this can put .pom files in the input files, so we need to remove those.
+                        .matching(filter -> {
+                            filter.include(fileTreeElement -> fileTreeElement.isDirectory() ||
+                                    fileTreeElement.getName().endsWith(".jar") ||
+                                    fileTreeElement.getName().endsWith(".zip"));
+                        })
+                        .getFiles().stream()
                         .map(File::getAbsolutePath)
                         .sorted()
                         .collect(Collectors.toCollection(LinkedHashSet::new)),