fix(v2): fix webpack SSG plugin error thrown due to new URL() / __filename

[slorber]

Motivation

Quite complicated to explain, fixes #4922 (including all the relevant details)

Not sure it's the perfect fix, but it looks to me we can pass any __filename value to unlock the situation, and it has no particular effect in practice apart preventing the eval error

Have you read the Contributing Guidelines on pull requests?

yes

Test Plan

local test + feedback from Relay Compiler playground integration, using Rust/WASM compiler

[netlify]

✔️ [V2]

🔨 Explore the source changes: efa3c76

🔍 Inspect the deploy log: https://app.netlify.com/sites/docusaurus-2/deploys/60d59d5b4253230007dc35d8

😎 Browse the preview: https://deploy-preview-5059--docusaurus-2.netlify.app

[github-actions]

⚡️ Lighthouse report for the changes in this PR:

Category	Score
🟢 Performance	96
🟢 Accessibility	98
🟢 Best practices	100
🟢 SEO	100
🟢 PWA	95

Lighthouse ran on https://deploy-preview-5059--docusaurus-2.netlify.app/

[github-actions]

Size Change: 0 B

Total Size: 601 kB

ℹ️ View Unchanged

Filename	Size
website/build/assets/css/styles.********.css	87.7 kB
website/build/assets/js/main.********.js	415 kB
website/build/blog/2017/12/14/introducing-docusaurus/index.html	66.5 kB
website/build/docs/introduction/index.html	235 B
website/build/index.html	31.3 kB

_{compressed-size-action}