Prefix internal modules with internal-* (tailwindlabs#14074)

Last week we discussed bringing in some consistency for our non-public
npm packages in the repo. We discussed using custom namespaces (e.g.
`@tailwindcss-internal`) vs. simple prefixes but it does not matter too
much if we are both consistent with our pattern and it's easy for us to
see whether a plugin is public or not.

Since we have a mixture of public namespaced (`@tailwindcss/*`) and
non-namespaced (`tailwindcss`) packages, I think it would be best if we
use a prefix to signal internal dependencies. This PR proposes we use
`internal-*` as the prefix and renames `test-utils` to
`internal-example-plugin` (since, really, this package is just an
example for the Tailwind plugin system).

Author: philipp-spiess

packages/@tailwindcss-postcss/package.json

@@ -39,6 +39,6 @@
     "@types/node": "^20.12.12",
     "@types/postcss-import": "^14.0.3",
     "postcss": "8.4.24",
-    "tailwindcss-test-utils": "workspace:*"
+    "internal-example-plugin": "workspace:*"
   }
 }

packages/@tailwindcss-postcss/src/index.test.ts

@@ -144,7 +144,7 @@ describe('plugins', () => {
     let result = await processor.process(
       css`
         @import 'tailwindcss/utilities';
-        @plugin 'tailwindcss-test-utils';
+        @plugin 'internal-example-plugin';
       `,
       { from: INPUT_CSS_PATH },
     )
@@ -174,7 +174,7 @@ describe('plugins', () => {
     let result = await processor.process(
       css`
         @import 'tailwindcss/utilities';
-        @plugin 'tailwindcss-test-utils';
+        @plugin 'internal-example-plugin';
       `,
       { from: INPUT_CSS_PATH },
     )

packages/test-utils/index.js renamed to packages/internal-example-plugin/index.js

@@ -1,5 +1,5 @@
 {
-  "name": "tailwindcss-test-utils",
+  "name": "internal-example-plugin",
   "version": "0.0.0",
   "private": true,
   "main": "index.js"