Update `rules_spm` to build fetched dependencies using Bazel instead of Swift Package Manager

[cgrindel]

Goals

• Fetch dependencies using Swift Package Manager (SPM)
• Generate Bazel build targets for the fetched dependencies
• Ensure that the fetched dependencies have typical Bazel dependency names (e.g. https://github.com/grpc/grpc-swift becomes com_github_grpc_grpc_swift)
• Provide a mechanism for external dependencies to have a custom Bazel dependency name.

Reasoning

• Address duplicate symbol errors when the client project and a client project dependency both load the same dependency
  • #148
  • #86
• Allow target platform compilation to work properly (e.g., build for iOS device on macOS)

Tasks

• [x] Add support for pure Swift packages
• [x] Add support for packages with C that do not have special settings (e.g. CSetting).
• [x] Derive source package directory name from the repository URL, not the package name.
• [ ] Migrate spm_parser to a separate repository.
  • [ ] Migrate the code
  • [ ] Create a release process that builds binaries for MacOS and Linux
• [ ] Update rules_spm to download spm_parser.
• [ ] Add code to interesting_deps example that uses libwebp. example
• [ ] Add support for packages with Objective-C.
• [ ] Add support for packages with C that do have special settings (e.g. CSetting).
• [ ] Implement functional tests for spm_parser.
• [ ] Enable integration tests that test build_mode set to bazel.

Working Examples

• [X] simple
• [X] simple_with_dev_dir
• [X] simple_revision
• [x] interesting_deps - Need to derive the source package name from the repo URL and not the pkg_name.
• [ ] simple_with_binary - The binary that we define works. The swiftformat and swiftlint do not work due to special C settings.
• [ ] ios_sim - Requires support for special C settings.
• [ ] local_package - Requires support for special C settings.
• [ ] public_hdrs - Need to find source path from path setting in the Package.swift. (e.g. TrustKit)
• [ ] vapor - Requires support for special C settings.