Support relative paths in hydra-build-products to enable relocatable derivations

# Support relative paths in hydra-build-products to enable relocatable derivations **Is your feature request related to a problem? Please describe.** Currently, hydra-build-products requires absolute store paths in the path field. This prevents derivation outputs that are perfectly relocatable from verifying this fact, as specifying build products forces them to contain self-references. For example, a derivation output at `/nix/store/abc-foo` must write: ``` doc none /nix/store/abc-foo/manual.pdf ``` This absolute path creates a self-reference in the output, making it non-relocatable even if the output would otherwise be position-independent. The validation in `src/hydra-queue-runner/build-result.cc:88` explicitly rejects non-absolute paths: ```cpp if (product.path == "" || product.path[0] != '/') continue; ``` **Describe the solution you'd like** Support relative paths in hydra-build-products that are resolved relative to the output path. For example: ``` doc none manual.pdf ``` This would be resolved to `$out/manual.pdf` internally by Hydra, allowing the derivation to remain relocatable while still declaring its build products. Alternatively, support an explicit output-relative syntax like: ``` doc none $out/manual.pdf ``` **Describe alternatives you've considered** 1. **Status quo**: Continue requiring absolute paths, accepting that all derivations with build products will have self-references 2. **Store-path-relative paths**: Allow paths like `abc-foo/manual.pdf` that are relative to the store directory, though this still encodes the hash 3. **Special sentinel paths**: Use a placeholder like `@OUTPUT@/manual.pdf` that Hydra replaces, though this is more complex **Additional context** This issue affects reproducibility and content-addressability efforts, where minimizing self-references is important for enabling features like: - Content-addressed derivations that can be deduplicated across different store paths - Binary cache sharing where outputs might be relocated - Verification that outputs are truly position-independent Related code: `src/hydra-queue-runner/build-result.cc:84-90` cc @Ericson2314