Anchor files to the package’s root

npm pack bases paths from the `files` array from the package’s
root. This updates yarn pack to behave in the same way.

__tests__/commands/pack.js

@@ -67,6 +67,18 @@ test.concurrent('pack should include all files listed in the files array', (): P
   });
 });
 
+test.concurrent('pack should include files based from the package’s root', (): Promise<void> => {
+  return runPack([], {}, 'files-include-from-root', async (config): Promise<void> => {
+    const {cwd} = config;
+    const files = await getFilesFromArchive(
+      path.join(cwd, 'files-include-from-root-v1.0.0.tgz'),
+      path.join(cwd, 'files-include-from-root-v1.0.0'),
+    );
+    expect(files.indexOf('index.js')).toBeGreaterThanOrEqual(0);
+    expect(files.indexOf('sub/index.js')).toEqual(-1);
+  });
+});
+
 test.concurrent('pack should included globbed files', (): Promise<void> => {
   return runPack([], {}, 'files-glob', async (config): Promise<void> => {
     const {cwd} = config;

__tests__/fixtures/pack/files-include-from-root/index.js

@@ -0,0 +1 @@
+console.log('included');

__tests__/fixtures/pack/files-include-from-root/package.json

@@ -0,0 +1,6 @@
+{
+  "name": "files-include-from-root",
+  "version": "1.0.0",
+  "license": "MIT",
+  "files": ["index.js"]
+}

__tests__/fixtures/pack/files-include-from-root/sub/index.js

@@ -0,0 +1 @@
+console.log('not included');

src/cli/commands/pack.js

@@ -85,8 +85,8 @@ export async function packTarball(
       '*', // ignore all files except those that are explicitly included with a negation filter
     ];
     lines = lines.concat(
-      onlyFiles.map((filename: string): string => `!${filename}`),
-      onlyFiles.map((filename: string): string => `!${path.join(filename, '**')}`),
+      onlyFiles.map((filename: string): string => `!${path.join('/', filename)}`),
+      onlyFiles.map((filename: string): string => `!${path.join('/', filename, '**')}`),
     );
     const regexes = ignoreLinesToRegex(lines, '.');
     filters = filters.concat(regexes);