feat: support customizing generated BUILD files for Swift packages

## Summary We've received multiple requests for the ability to customize or override the BUILD files that are generated for Swift packages. Currently, the auto-generated BUILD files cannot be modified by users, and even patched-in BUILD files may not survive the generation process. ## Use Cases 1. **Override/customize generated BUILD files** — Modify or replace what gets auto-generated for a package (e.g., to support building a package in multiple configurations with different environment variables). 2. **Add BUILD files that aren't generated** — Add a BUILD file to a package subdirectory that Gazelle doesn't generate one for (e.g., to export a specific file). ## Prior Discussion - https://bazelbuild.slack.com/archives/C05NZSHE7FU/p1774644485816069 - https://bazelbuild.slack.com/archives/C05NZSHE7FU/p1773254132884349 ## Looking for Feedback We'd like community input on the desired behavior and approach. Some questions to consider: - Should we support `build_file` / `build_file_content` attributes (similar to `http_archive`) to fully replace a generated BUILD file? - Should we support a post-generation overlay/patch mechanism that merges user-provided content with the generated BUILD file? - Should we support both adding new BUILD files and modifying/replacing existing generated ones? - Are there other approaches or patterns from the Bazel ecosystem that would work well here? If you have a use case for this feature, please share it here so we can design a solution that covers the common scenarios.