Fix @kbn/import-resolver detection of nested node_modules (#162391)

Ensure that a plugin containing a nested `node_modules` folder correctly
identifies imports from that folder as regular node modules instead of relative imports. 

Kibana should currently only have a single root `node_modules` folder, so this should
not be a real issue. However, if for some reason a stray `node_modules` folder is
created, this commit will make sure it's not introducing hard to debug ESLint errors.

.gitignore

@@ -11,6 +11,7 @@ node_modules
 !/src/dev/npm/integration_tests/__fixtures__/fixture1/node_modules
 !/src/dev/notice/__fixtures__/node_modules
 !/packages/kbn-import-resolver/src/__fixtures__/node_modules
+!/packages/kbn-import-resolver/src/__fixtures__/packages/box/node_modules
 trash
 /optimize
 /built_assets

packages/kbn-import-resolver/src/import_resolver.ts

@@ -86,7 +86,7 @@ export class ImportResolver {
 
   getPackageIdForPath(path: string) {
     const relative = Path.relative(this.cwd, path);
-    if (relative.startsWith('..')) {
+    if (relative.startsWith('..') || path.includes(NODE_MODULE_SEG)) {
       return null;
     }
 

packages/kbn-import-resolver/src/integration_tests/import_resolver.test.ts

@@ -42,6 +42,17 @@ describe('#resolve()', () => {
     `);
   });
 
+  it('resolves nested node_module imports', () => {
+    expect(resolver.resolve('bar', Path.join(FIXTURES_DIR, 'packages', 'box')))
+      .toMatchInlineSnapshot(`
+      Object {
+        "absolute": <absolute path>/packages/kbn-import-resolver/src/__fixtures__/packages/box/node_modules/bar/index.js,
+        "nodeModule": "bar",
+        "type": "file",
+      }
+    `);
+  });
+
   it('resolves requests to src/', () => {
     expect(resolver.resolve('src/core/public', FIXTURES_DIR)).toMatchInlineSnapshot(`
       Object {