Support dynamic linking on Linux via Artifactbundles

[tayloraswift]

proof-of-concept for enabling self-managed dynamic library dependencies on Linux!

Motivation:

see Forums discussion, #5714

Modifications:

• adds an ArtifactType called “library”, which can be used in info.json inside an Artifactbundle. variant/triple checking is not implemented yet, and we expect this to be very limited and geared towards self-managed deployment use cases, as we intentionally do not want to design a compatibility checking system that covers all possible host–target combinations of “Linux”.

{
    "schemaVersion": "1.0",
    "artifacts": {
        "MyLibrary": {
            "type": "library",
            "version": "0.0.0",
            "variants": [{ "path": "MyLibrary" }]
        }
    }
}

• adds logic to Sources/Build/BuildPlan/BuildPlan+Swift.swift that injects the necessary .swiftmodule search paths to the swiftc build command.

• adds logic to Sources/Build/BuildPlan/BuildPlan+Product.swift that populates the necessary linker flags to compile products that include dependencies on non-XCFramework binary targets

• does a small amount of refactoring around the additions i made, in an effort to leave the code base in a slightly better shape than i found it in

Result:

SwiftPM can now dynamically link binary library dependencies into executables on Linux, although the executables of course will require the shared libraries to be installed in the expected locations to actually run.

sample manifest:

// swift-tools-version:6.0
import PackageDescription

let package = Package(
    name: "DynamicLinkingTest",
    products: [
        // .library(name: "MyLibrary", type: .dynamic, targets: ["MyLibrary"]),
        .executable(name: "Client", targets: ["Client"]),
    ],
    dependencies: [
    ],
    targets: [
        .binaryTarget(
            name: "MyLibrary",
            path: "test.artifactbundle"),

        .executableTarget(name: "Client",
            dependencies: [
                .target(name: "MyLibrary"),
            ]),

        // .target(name: "MyLibrary",
        //     swiftSettings: [
        //         .unsafeFlags(["-enable-library-evolution"]),
        //     ]),
    ]
)

sample Artifactbundle

- test.artifactbundle
    - MyLibrary
        - libMyLibrary.so
        - MyLibrary.swiftmodule
    - info.json