SwiftPM lowering omits sibling path-package dependencies

Summary

When a Swift package depends on a sibling package through .package(path:), Once loads the root package but omits the sibling package’s products from the derived dependency edges. The graph validates successfully, then compilation fails when source files import the missing module.

Reproduction

Given this layout:

repo/
  PackageA/
    Package.swift
    Sources/
  PackageB/
    Package.swift
    Sources/

PackageA declares:

.package(path: "../PackageB")

A macro target in PackageA depends on a product from PackageB:

.product(name: "PackageBMacroSupport", package: "PackageB")

From PackageA:

once query target SwiftPackage_PackageA_PackageAMacros
once query validate-workspace
once build SwiftPackage_PackageA_PackageA

The target record does not include PackageBMacroSupport in deps, and workspace validation reports no diagnostics. Compilation later fails with:

error: no such module 'PackageBMacroSupport'

The same omission affects ordinary library products from sibling path packages, not only macro support targets.

Expected behavior

Once should follow declared local path dependencies and include their selected products in the graph. If dependencies outside the selected workspace root are intentionally unsupported, graph loading should report that boundary instead of returning a valid but incomplete graph.

Tested with Once 0.57.0 on macOS.

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