Issue Description:

I am developing an app which is a Swift package. Building it using swift build works completely fine. However, when I include it as a dependency in another package, building that package fails with the following error:

invalidManifestFormat("'./Sources/Core' is not a valid path for path-based dependencies; use relative or absolute path instead.", diagnosticFile: nil) in https://github.com/stackotter/delta-client.

The path that is causing the error is valid when building the delta-client package, but not when delta-client is a dependency.

Here is an example Package.swift that results in the aforementioned error:

// swift-tools-version:5.5

import PackageDescription

let package = Package(
  name: "BugReproducer",
  dependencies: [
    .package(name: "DeltaClient", url: "https://github.com/stackotter/delta-client", .branch("refactor-rendering")),
  ],
  targets: [
    .executableTarget(
      name: "BugReproducer",
      dependencies: [
        .product(name: "StaticShim", package: "DeltaClient")
      ]),
  ]
)