Revert "Move FileExtensions into PluginSpecification"

This reverts commit e87fd0c.

src/funcTest/groovy/com/github/jengelman/gradle/plugins/shadow/util/FileExtensions.groovy

@@ -0,0 +1,26 @@
+package com.github.jengelman.gradle.plugins.shadow.util
+
+/**
+ * TODO: this is used as extensions for Groovy, could be replaced after migrated to Kotlin.
+ *  Registered in resources/META-INF/services/org.codehaus.groovy.runtime.ExtensionModule.
+ */
+final class FileExtensions {
+    static final File resolve(File file, String relativePath) {
+        try {
+            return new File(file, relativePath)
+        } catch (RuntimeException e) {
+            throw new RuntimeException(String.format("Could not locate file '%s' relative to '%s'.", Arrays.toString(relativePath), file), e)
+        }
+    }
+
+    static final File createDir(File file) {
+        if (file.mkdirs()) {
+            return file
+        }
+        if (file.isDirectory()) {
+            return file
+        }
+        throw new AssertionError("Problems creating dir: " + file
+            + ". Diagnostics: exists=" + file.exists() + ", isFile=" + file.isFile() + ", isDirectory=" + file.isDirectory())
+    }
+}

src/funcTest/groovy/com/github/jengelman/gradle/plugins/shadow/util/PluginSpecification.groovy

@@ -185,29 +185,4 @@ abstract class PluginSpecification extends Specification {
         }
         return new File(gradleUserHome, "testkit")
     }
-
-    /**
-     * TODO: this is used as extensions for Groovy, could be replaced after migrated to Kotlin.
-     *  Registered in resources/META-INF/services/org.codehaus.groovy.runtime.ExtensionModule.
-     */
-    static final class FileExtensions {
-        static final File resolve(File file, String relativePath) {
-            try {
-                return new File(file, relativePath)
-            } catch (RuntimeException e) {
-                throw new RuntimeException(String.format("Could not locate file '%s' relative to '%s'.", Arrays.toString(relativePath), file), e)
-            }
-        }
-
-        static final File createDir(File file) {
-            if (file.mkdirs()) {
-                return file
-            }
-            if (file.isDirectory()) {
-                return file
-            }
-            throw new AssertionError("Problems creating dir: " + this
-                + ". Diagnostics: exists=" + file.exists() + ", isFile=" + file.isFile() + ", isDirectory=" + file.isDirectory())
-        }
-    }
 }

src/funcTest/resources/META-INF/services/org.codehaus.groovy.runtime.ExtensionModule

@@ -1,3 +1,3 @@
 moduleName = groovy-extensions
 moduleVersion = ${moduleVersion}
-extensionClasses =com.github.jengelman.gradle.plugins.shadow.util.PluginSpecification.FileExtensions
+extensionClasses =com.github.jengelman.gradle.plugins.shadow.util.FileExtensions