Issue Description:

I have a local dynamic library. Let's call it MyLib.dylib.

I want to wrap some functionality in SPM, but testTarget in this package requires MyLib.dylib to be loaded.

I can copy this library into Resources of test target bundle, but I can't set runpath to it. Is it possible to set runpath?..

A part of package:

```swift
.target: (name: "MyTarget"),
.testTarget: (
name: "MyTargetTests",
resources: [
"Resources/MyLib.dylib" // A copy of this library in Resources. It will be copied
],
linkerSettings: [
.unsafeFlags([
"-L",
"../../LibraryPath" // A link to an actual library location
"-l",
"MyLib.dylib",
"-rpath",
"@loader_path/../Resources"
])
]
)

```

Notes

Yes, I have a cheat here, I put a link to a folder ("../../LibraryPath") with this library into a package's root, so, compiler can find it.
But I don't understand how to set runpath for this target that linker can load it correctly.