Turbopack + Relay: correctly implement artifact_directory to fix multi-project test (#67403)
Previously, if `artifact_directory` were set, artifacts would be
resolved at a fixed location relative to each file. This correctly
resolves them from the project root.
Test Plan: `TURBOPACK_BUILD=1 TURBOPACK=1 pnpm test-dev
test/integration/relay-graphql-swc-multi-project/test/index.test.js`
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Co-authored-by: Tobias Koppers <tobias.koppers@googlemail.com>