Issue Description:

I have a package for a library, swift-url. Within its repository, I have a folder "Benchmarks", containing another Swift package. The Package.swift for Benchmarks looks as follows:

// swift-tools-version:5.3
import PackageDescription

let package = Package(
    name: "swift-url-benchmark",
    products: [
        .executable(name: "WebURLBenchmark", targets: ["WebURLBenchmark"])
    ],
    dependencies: [
      .package(name: "Benchmark", url: "https://github.com/google/swift-benchmark", from: "0.1.0"),
      .package(name: "swift-url", path: ".."),
    ],
    targets: [
      .target(
        name: "WebURLBenchmark",
        dependencies: [
          .product(name: "WebURL", package: "swift-url"),
          .product(name: "Benchmark", package: "Benchmark")
        ]
      )
    ]
)

Where the "WebURL" product is defined by swift-url's Package.swift.

swift-url builds, and its tests build, run, and pass on macOS, Linux, and Windows. Benchmarks fails to build on Windows only, and it seems like the issue is the relative path.

On macOS and Linux, I can build and run WebURLBenchmark even if I omit naming the parent package as "swift-url" in the package dependencies list (just writing `.package(path: "..")`) even if I refer to it by name in the target dependencies. SwiftPM somehow figures it out from the parent folder's name. If I try that on Windows, I get the following error:

D:\a\swift-url\swift-url\Benchmarks: error: unknown package 'swift-url' in dependencies of target 'WebURLBenchmark'; valid packages are: 'Benchmark', 'D:'
Error: Process completed with exit code 1.

"D:" is obviously not my package name. Seems like some path issues. If I add an explicit package name in the package dependencies list (so it looks like the snippet I pasted above), the error becomes:

Running resolver because the following dependencies were added: 'https://github.com/google/swift-benchmark', 'D:\'
error: D:\Package.swift has no Package.swift manifest
Error: Process completed with exit code 1.