refactor: move plugin tests to internal

Match the structure of the `./src` folder in `./tests` for the internal
plugins tests.

Author: kanej

v-next/core/test/plugins/fixture-projects/installed-package/README.md v-next/core/test/internal/plugins/fixture-projects/installed-package/README.md

@@ -1,8 +1,8 @@
 import assert from "node:assert/strict";
 import { describe, it } from "node:test";
 
-import { validatePluginNpmDependencies } from "../../src/internal/plugins/plugin-validation.js";
-import { HardhatPlugin } from "../../src/types/plugins.js";
+import { validatePluginNpmDependencies } from "../../../src/internal/plugins/plugin-validation.js";
+import { HardhatPlugin } from "../../../src/types/plugins.js";
 
 describe("Plugins - plugin validation", () => {
   const plugin: HardhatPlugin = {

v-next/core/test/plugins/fixture-projects/installed-package/node_modules/example/package.json v-next/core/test/internal/plugins/fixture-projects/installed-package/node_modules/example/package.json

@@ -1,8 +1,8 @@
 import assert from "node:assert/strict";
 import { describe, it } from "node:test";
 
-import { resolvePluginList } from "../../src/internal/plugins/resolve-plugin-list.js";
-import { HardhatPlugin } from "../../src/types/plugins.js";
+import { resolvePluginList } from "../../../src/internal/plugins/resolve-plugin-list.js";
+import { HardhatPlugin } from "../../../src/types/plugins.js";
 
 describe("Plugins - resolve plugin list", () => {
   const installedPackageFixture = import.meta.resolve(