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Issue Description:

The `url(forResource:withExtension:subdirectory:localization: )` resource location method is (at best) inconsistent across platforms and can (at worst) cause a "core dumped" error when used with a Swift Package which includes resources.

The attached `CLIQuickstartLib` project is a standalone, very-close-to-minimal Swift Package which is set up to evaluate access approaches to package resources. Resources are included for each of the targets: library, executable and tests.

The resource access evaluation scenarios factors include:

• platform: Linux, macOS

• build chain execution: Linux command line interface (CLI), macOS command line interface (CLI), macOS Xcode IDE (open from Package.swift directly, not an `generate-xcodeproj`)

• top level resources directory name: `Resources` or something else

• resource asset directory level: top level or some sub directory

• use of localization

In particular, the `checkResourceAccessApproaches()` method in `SpmHelper.swift` partially enumerates various ways that `url(forResource:withExtension:…)` might be called to access package resources.

In general, `url(forResource: …)` is expected to reliably handle cross-platform differences in a reliably consistent and robust way. However, it does not.

However, here is a table of what was observed:

⚑ Fatal Runtime Error (Linux CLI)

# Foundation/NSCFString.swift:119: Fatal error: Constant strings cannot be deallocated
# Illegal instruction (core dumped)

Issues

• Top level resource assets can not accessed in a uniform, reliable way across platforms. (i.e. without using #if os(…))

• Top level resource directory name affects how assets can be successfully accessed.

• In some cases, Swift Foundation on Linux exhibits a fatal "core dumped" runtime error. (this really should fail more gracefully!)

• Localized files cannot be access in the same way on the Linux and macOS command build/test/run.

• Localized files cannot be access via the localization argument if `Package.swift` is opened with Xcode.

Replication Steps

Run the `CLIQuickstartLib` package on the macOS and Linux terminal command line.

 swift build
 swift run
 swift test
 

For Xcode, open Package.swift directly. Do not use `generate-xcodeproj`. Set the schema to "CLIQuickstartTool > My Mac". Use the Xcode menus "Product > Build", "Product > Run", and "Product > Test"

Look at the console results in all cases.