fix(cli): Swift package plugin product graph edges

Fixes #11453.

What changed

This changes the external dependency resolver so plugin products from Swift Package Manager packages are kept as resolvable product names, but map to an empty dependency list instead of framework or project dependencies.

It also adds regression coverage for a plugin-only package product.

Why

Packages can depend on build-tool plugins transitively. Those plugin targets are intentionally not generated as Xcode projects, but consumers can still see the plugin product during external dependency resolution.

Root cause

PackageInfoMapper.resolveExternalDependencies treated plugin products like normal products and resolved their targets into project dependencies. Later project generation filters plugin targets out, so the generated graph could point to a project that does not exist and fail with “Could not locate project at path”.

Approach

Plugin products remain in the product dependency map to avoid treating them as unknown products. Their value is an empty dependency list because there is no runtime or linkable graph edge to generate for a build-tool plugin product.

Impact

tuist generate can handle external packages that depend transitively on build-tool plugin products without creating invalid project references. There is no migration impact.

Validation

• TUIST_CACHE_ENDPOINT=https://tuist.dev mise x -- tuist generate run tuist TuistLoader TuistLoaderTests ProjectDescription --no-open --no-binary-cache --cache-profile none
• xcodebuild test -workspace Tuist.xcworkspace -scheme Tuist-Workspace -only-testing 'TuistLoaderTests/PackageInfoMapperTests/resolveDependencies_whenProductIsPlugin_mapsToNoDependencies()' CODE_SIGNING_ALLOWED=NO CODE_SIGNING_REQUIRED=NO CODE_SIGN_IDENTITY="" COMPILATION_CACHE_ENABLE_CACHING=NO