Tree-shaking package imports that include file extensions

[FredKSchott]

Original Discussion: #965
/cc @Papierschiff

For example, create a new module test at the node_modules directory with one file, add one method to this file, export this method and use it in your code. I get the following behaviour for imports

import * as TestModule from 'test'; // is working
import * as TestModule from 'test/index'; // is working
import * as TestModule from 'test/index.js'; // is not working("Generated an empty chunk: "te

Bug is most likely coming from this line in src/rollup-plugins/rollup-plugin-wrap-install-targets.ts:

        const allInstallTargets = installTargets.filter(
          (imp) => imp.specifier.replace(/\.js$/, 'js') === key,
        );

We convert a .js -> js for packages like tippy.js that get installed as tippyjs.js. It looks like we need to restrict this to only a package name. In the past, we've done that by calling:

import validatePackageName from 'validate-npm-package-name';
if (validatePackageName(imp.specifier).validForNewPackages) {
  // this is a package name, not a path within a package

[zhusjfaker]

c.ts
export * from './a';
export * from './b';

index.ts
import { TestBComponent } from './c';

snowpack will install a.ts & b.ts
treeshake is not work