Avoid using import mapping in model generation (#11217)

* remove import mapping logic in model generation, better handle of file, list

* add new files for file, list

* fix some tests

* update tests, doc

* skip file as reserved word in feign client

* add new files

* remove file from reserved word list in spring generator

samples/client/petstore/java/feign/src/test/java/org/openapitools/client/model/FileTest.java

@@ -0,0 +1,48 @@
+/*
+ * OpenAPI Petstore
+ * This spec is mainly for testing Petstore server and contains fake endpoints, models. Please do not use this for any other purpose. Special characters: \" \\
+ *
+ * The version of the OpenAPI document: 1.0.0
+ * 
+ *
+ * NOTE: This class is auto generated by OpenAPI Generator (https://openapi-generator.tech).
+ * https://openapi-generator.tech
+ * Do not edit the class manually.
+ */
+
+
+package org.openapitools.client.model;
+
+import com.fasterxml.jackson.annotation.JsonInclude;
+import com.fasterxml.jackson.annotation.JsonProperty;
+import com.fasterxml.jackson.annotation.JsonCreator;
+import com.fasterxml.jackson.annotation.JsonTypeName;
+import com.fasterxml.jackson.annotation.JsonValue;
+import io.swagger.annotations.ApiModel;
+import io.swagger.annotations.ApiModelProperty;
+import org.junit.jupiter.api.Test;
+
+
+/**
+ * Model tests for File
+ */
+class FileTest {
+    private final File model = new File();
+
+    /**
+     * Model tests for File
+     */
+    @Test
+    void testFile() {
+        // TODO: test File
+    }
+
+    /**
+     * Test the property 'sourceURI'
+     */
+    @Test
+    void sourceURITest() {
+        // TODO: test sourceURI
+    }
+
+}