Support Webpack 5 by updating VirtualModulesPlugin from upstream

[syvb]

tl;dr: Webpack 5 support, fixes TypeError: Cannot read property 'data' of undefined

This updates the internal VirtualModulesPlugin, which is copied from webpack-virtual-modules, which was originally added by @kaisermann. These changes allow for using Webpack 5 with svelte-loader. The changes of #125, which have also been fixed upstream, have been preserved. See #59 (comment) for the background on why the source code is just copied over instead of added as a dependency. Due to the svelte-loader specific changes already in virtual.js, I have manually gone through the commit history of webpack-virtual-modules and applied relevant changes instead of just copy-pasting the new source code. There is also a small change to index.js to prevent the error handling code from itself throwing errors, since that was causing some (now fixed) troubles when debugging my changes.

This also adds in the literal text of the license of webpack-virtual-modules, since "The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software".

Tests and lints pass, and I have verified this works with both Webpack 4 and Webpack 5.

[benmccann]

For the license, let's just create a LICENSE file in the repo like https://github.com/sveltejs/svelte/blob/master/LICENSE instead of adding to each individual file.

[syvb]

@benmccann I've added a LICENSE file to the root, and updating the directory handling code with the changes from 2773097.

[benmccann]

thanks. this seems fine to me. I'll leave it for someone who's more familiar with this repo to do the merge and cut a release

[non25]

While changes from this PR hasn't been merged I ask you to integrate an important fix from upstream.
sysgears/webpack-virtual-modules@0c8926e

Context:
sysgears/webpack-virtual-modules#65
sysgears/webpack-virtual-modules#66
#131
#133

[syvb]

@non25 I've updated with that patch. Not sure why this got closed.

[benmccann]

@Smittyvb it says there are no code changes on the branch anymore, which is why I think it was closed

[syvb]

@benmccann It looks like GH will no longer let me re-open this, so see #136 for the continuation of this.