Update Yarn PnP to match the latest specification

[evanw]

Changes:

• Make $$virtual an alias for __virtual__
• Update testExpectations.json to the latest value
• Check for platform-specific absolute paths instead of always for the / prefix
• Ignore PnP manifests in virtual folders

With these changes, Yarn now builds with these instructions:

yarn up esbuild@0.15.0
sed -i.bak -e "s/, pnpPlugin()//" packages/yarnpkg-builder/sources/commands/build/bundle.ts
sed -i.bak -e "s/silent/debug/" packages/yarnpkg-builder/sources/commands/build/bundle.ts
ESBUILD_BINARY_PATH=<path-to-esbuild> yarn build:cli

Fixes #2452