Stop versioning Checkstyle IDE config (elastic#87285)

Closes elastic#86945.

The IntelliJ Checkstyle plugin likes to modify its configuration file
unecessarily, which creates friction for developers because they have
the revert these changes.

Instead, stop versioning the config file and instead update the
`configureIdeCheckstyle` task to copy the required content into place.

.gitignore

@@ -9,7 +9,6 @@ build-idea/
 out/
 
 # include shared intellij config
-!.idea/checkstyle-idea.xml
 !.idea/eclipseCodeFormatter.xml
 !.idea/externalDependencies.xml
 !.idea/inspectionProfiles/Project_Default.xml

build-tools-internal/src/main/groovy/elasticsearch.ide.gradle

@@ -25,15 +25,35 @@ tasks.register('configureIdeCheckstyle') {
   group = 'ide'
   description = 'Generated a suitable checkstyle config for IDEs'
 
-  String checkstyleConfig = 'build-tools-internal/src/main/resources/checkstyle.xml'
-  String checkstyleSuppressions = 'build-tools-internal/src/main/resources/checkstyle_suppressions.xml'
-  String checkstyleIdeFragment = 'build-tools-internal/src/main/resources/checkstyle_ide_fragment.xml'
-  String checkstyleIdeConfig = "$rootDir/checkstyle_ide.xml"
-
-  inputs.files(file(checkstyleConfig), file(checkstyleIdeFragment))
-  outputs.files(file(checkstyleIdeConfig))
+  String resources = 'build-tools-internal/src/main/resources'
+  String checkstyleConfig = "${resources}/checkstyle.xml"
+  String checkstyleSuppressions = "${resources}/checkstyle_suppressions.xml"
+  String checkstyleIdeFragment = "${resources}/checkstyle_ide_fragment.xml"
+  String checkstyleIdeConfig = "${rootDir}/checkstyle_ide.xml"
+
+  String checkstylePluginConfigTemplate = "${resources}/checkstyle-idea.xml"
+  String checkstylePluginConfig = "${rootDir}/.idea/checkstyle-idea.xml"
+
+  inputs.files(
+    file(checkstyleConfig),
+    file(checkstyleIdeFragment),
+    file(checkstylePluginConfigTemplate)
+  )
+  outputs.files(
+    file(checkstyleIdeConfig),
+    file(checkstylePluginConfig)
+  )
 
   doLast {
+    // Configure the IntelliJ Checkstyle plugin by copying a standard file. We don't simply commit
+    // the result to version control, because the plugin has a habit of modifying the file and
+    // replacing the `$PROJECT_DIR$` placeholders, which developers must then revert.
+    Files.copy(
+      Paths.get(file(checkstylePluginConfigTemplate).getPath()),
+      Paths.get(file(checkstylePluginConfig).getPath()),
+      StandardCopyOption.REPLACE_EXISTING
+    )
+
     // Create an IDE-specific checkstyle config by first copying the standard config
     Files.copy(
       Paths.get(file(checkstyleConfig).getPath()),