SwiftBuild: Fix executable can't find dynamic lib runtime error

[pmattos]

Update LD_RUNPATH_SEARCH_PATHS of executable built using the new Swift Build backend.

Motivation:

For the Swift Build backend, fix a runtime issue when building & running an executable linked against a dynamic library. Given the following packages:

Package App

let package = Package(
    name: "App",
    dependencies: [
        .package(path: "../Lib")
    ],
    targets: [
        .executableTarget(
            name: "App",
            dependencies: [
                .product(name: "Utils", package: "Lib")
            ]
        )
    ]
)

Package Lib

let package = Package(
    name: "Lib",
    products: [
        .library(
            name: "Utils",
            type: .dynamic,
            targets: ["Utils"]
        )
    ],
    targets: [
        .target(name: "Utils")
    ]
)

This the error that I get:

$ swift run --package-path /App --build-system swiftbuild

dyld[89079]: Library not loaded: @rpath/libUtils.dylib
Referenced from: 
<98AF2F50-2C62-318C-8430-C6326D8468F1> /App/.build/arm64-apple-macosx/Products/Debug/App
Reason: tried: 
'/usr/lib/swift/libUtils.dylib' (no such file, not in dyld cache), 
'/App/.build/arm64-apple-macosx/Products/Debug/PackageFrameworks/libUtils.dylib' (no such file),
'/System/Volumes/Preboot/Cryptexes/OS/usr/lib/swift/libUtils.dylib' (no such file), 
'/System/Volumes/Preboot/Cryptexes/OS/App/.build/arm64-apple-macosx/Products/Debug/PackageFrameworks/libUtils.dylib' (no such file),

...but works just fine when using --build-system native.

When comparing the runpath, we get this:

# Swift Build
.build/arm64-apple-macosx/Products/Debug/App
/usr/lib/swift
/Users/pmattos/SamplePackages/App+DynamicLib/App/.build/arm64-apple-macosx/Products/Debug/PackageFrameworks

# Native
.build/arm64-apple-macosx/debug/App
/usr/lib/swift
@loader_path
/Applications/Xcode_17A233.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib/swift-5.5/macosx

PS. To get the runpath, run otool -l path/to/bin | awk '/LC_RPATH/ {getline; getline; print $2}'.

Modifications:

Update the build setting LD_RUNPATH_SEARCH_PATHS for executables, appending @loader_path. This is actually an "imparted" build setting coming from dynamic libraries targets, so that's where I made the change.

Result:

Now both configurations are capable of running the built App:

$ swift run --package-path /App --build-system swiftbuild --configuration debug
Hello from Lib

$ swift run --package-path /App --build-system swiftbuild --configuration release
Hello from Lib

For reference, this is the PIF for the sample package App above. Note my change affects the bottom PACKAGE-TARGET:Utils target only. The build setting is then "imparted" on the top PACKAGE-PRODUCT:App target.

[pmattos]

@swift-ci test

[jakepetroules]

This is changing the behavior a bit from previously since the settings are no longer being added to a separate debugImpartedSettings variable. Since $(BUILT_PRODUCTS_DIR)/PackageFrameworks is an absolute path to the build outputs, that should only be present in debug configurations, not in release configurations.

@loader_path on the other hand should be present in both places.