Relative path references in Package.swift dependencies do not work with Skip Fuse

[marcprux]

Consider a package uses a local dependency "SomeLocalDependency" referenced by a relative path to the Package.swift file:

// swift-tools-version: 6.1
// This is a Skip (https://skip.tools/) package.
import PackageDescription

let package = Package(
    name: "my-fuse-project",
    products: [
        .library(name: "MyProject", type: .dynamic, targets: ["MyProject"]),
    ],
    dependencies: [
        .package(url: "https://source.skip.tools/skip.git", from: "1.6.5"),
        .package(url: "https://source.skip.tools/skip-fuse-ui.git", from: "1.0.0"),
        .package(name: "SomeLocalDependency", path: "../../Packages/SomeLocalDependency")
    ],
    targets: [
        .target(
            name: "MyProject",
            dependencies: [
                .product(name: "SkipFuseUI", package: "skip-fuse-ui"),
                .product(name: "Alamofire", package: "Alamofire"),
                .byName(name: "SomeLocalDependency")
            ],
            plugins: [.plugin(name: "skipstone", package: "skip")]
        ),
    ]
)

This is a valid Swift package, but when building for Android as part of a Skip application, the build will fail with an error like:

FAILURE: Build failed with an exception.

* What went wrong:
Could not determine the dependencies of task ':app:compileDebugJavaWithJavac'.
> Could not resolve all dependencies for configuration ':app:debugCompileClasspath'.
   > Could not resolve project :skipstone: SomeLocalDependency.
     Required by:
         project :app > project :skipstone:Bricksee
      > No matching variant of project :skipstone: SomeLocalDependency was found. 

The issue is that Skip Fuse's native Android build creates a derived Package.swift in a different location because we need to add some code to the Package.swift file, but the fact that it is being built from a separate folder means that relative path references are no longer valid.

The best workaround (aside from pushing the repository somewhere and referencing it through a git https) is to change the relative path to an absolute path. E.g.:

.package(name: "SomeLocalDependency", path: "../../Packages/SomeLocalDependency")

would become:

.package(name: "SomeLocalDependency", path: "/path/to/project/Packages/SomeLocalDependency")

One thing we might do is at least raise a warning or error when we detect that the package uses a relative path. Another option could be to actually identify when the package is using a relative path and re-write it to use the absolute path equivalent.