Add System.FilePath <-> SystemPackage.FilePath conversion?

It's quite awkward to work on an application where both System.FilePath and SystemPackage.FilePath are floating around.

I _love_ that this package is available, but as things stand, I'm having to add the following code to my libraries to facilitate type conversions:

```swift
#if canImport(System)
import System
import SystemPackage

@available(macOS 11, iOS 14, tvOS 14, watchOS 7, *)
extension System.FilePath {

/// Creates a `FilePath` (from the platform's `System` framework) from a `FilePath` (from the `swift-system` package).
///
public init(_ packageFilePath: SystemPackage.FilePath) {
self = packageFilePath.withCString { .init(cString: $0) }
}
}

@available(macOS 11, iOS 14, tvOS 14, watchOS 7, *)
extension SystemPackage.FilePath {

/// Creates a `FilePath` (from the `swift-system` package) from a `FilePath` (from the platform's `System` framework).
///
public init(_ sdkFilePath: System.FilePath) {
self = sdkFilePath.withCString { .init(cString: $0) }
}
}
#endif
```

But this isn't really the kind of code I feel my library should be responsible for. It's really none of my business to bridge between these 2 libraries. Would it be acceptable to add these to the package distribution?

I understand that the compiler may provide better tools for dealing with this, and if/when that happens, these functions could be deprecated and removed at the next SemVer-major release. Until that time, adding these functions (and perhaps others for `FileDescriptor`) would make using the package distribution much more ergonomic.

Start by reviewing the System.FilePath and SystemPackage.FilePath APIs described in the issue, including their availability and conversion behavior. Determine the supported scope for package-provided conversions and add focused coverage for supported platforms; done means clients can convert in both directions without local bridging code.

Written by the indexing model from the issue text.