always migrate the js config in the Tailwind Root file

We already computed the correct Tailwind root file (and linked the JS
config to it). This means that we can remove the logic in the
migrate-config migration to find the correct location.

Author: RobinMalfait

packages/@tailwindcss-upgrade/src/codemods/migrate-config.ts

@@ -1,5 +1,5 @@
 import path from 'node:path'
-import postcss, { AtRule, type Plugin, Root } from 'postcss'
+import postcss, { AtRule, type Plugin } from 'postcss'
 import { normalizePath } from '../../../@tailwindcss-node/src/normalize-path'
 import type { JSConfigMigration } from '../migrate-js-config'
 import type { Stylesheet } from '../stylesheet'
@@ -13,7 +13,9 @@ export function migrateConfig(
     jsConfigMigration,
   }: { configFilePath: string; jsConfigMigration: JSConfigMigration },
 ): Plugin {
-  function injectInto(sheet: Stylesheet) {
+  function migrate() {
+    if (!sheet.isTailwindRoot) return
+
     let alreadyInjected = ALREADY_INJECTED.get(sheet)
     if (alreadyInjected && alreadyInjected.includes(configFilePath)) {
       return
@@ -82,41 +84,6 @@ export function migrateConfig(
     root.append(cssConfig.nodes)
   }
 
-  function migrate(root: Root) {
-    // We can only migrate if there is an `@import "tailwindcss"` (or sub-import)
-    let hasTailwindImport = false
-    let hasFullTailwindImport = false
-    root.walkAtRules('import', (node) => {
-      if (node.params.match(/['"]tailwindcss['"]/)) {
-        hasTailwindImport = true
-        hasFullTailwindImport = true
-        return false
-      } else if (node.params.match(/['"]tailwindcss\/.*?['"]/)) {
-        hasTailwindImport = true
-      }
-    })
-
-    if (!hasTailwindImport) return
-
-    // If a full `@import "tailwindcss"` is present or this is the root
-    // stylesheet, we can inject the `@config` directive directly into this
-    // file.
-    if (hasFullTailwindImport || sheet.parents.size <= 0) {
-      injectInto(sheet)
-      return
-    }
-
-    // Otherwise, if we are not the root file, we need to inject the `@config`
-    // into the root file.
-    if (sheet.parents.size > 0) {
-      for (let parent of sheet.ancestors()) {
-        if (parent.parents.size === 0) {
-          injectInto(parent)
-        }
-      }
-    }
-  }
-
   return {
     postcssPlugin: '@tailwindcss/upgrade/migrate-config',
     OnceExit: migrate,