Declare a package name for CMake builds and remove @_spi(ForSwiftTestingOnly) (#954)

This removes usage of `@_spi(ForSwiftTestingOnly)` throughout the
codebase and adjusts the CMake build rules to allow adoption of Swift's
`package` access level. The technical constraints which prevented
adopting that feature have been resolved.

### Checklist:

- [x] Code and documentation should follow the style of the [Style
Guide](https://github.com/apple/swift-testing/blob/main/Documentation/StyleGuide.md).
- [x] If public symbols are renamed or modified, DocC references should
be updated.

Author: stmontgomery

Documentation/SPI.md

@@ -39,14 +39,6 @@ external tools, _both_ groups are specified. Such SPI is not generally meant to
 be promoted to public API, but is still experimental until tools authors have a
 chance to evaluate it.
 
-For interfaces internal to Swift Testing that must be available across targets,
-the SPI group `@_spi(ForSwiftTestingOnly)` is used. They _should_ be marked
-`package` and may be in the future, but are currently exported due to technical
-constraints when Swift Testing is built using CMake.
-
-> [!WARNING]
-> Never use symbols marked `@_spi(ForSwiftTestingOnly)`.
-
 ## SPI stability
 
 The testing library does **not** guarantee SPI stability for either group of
@@ -59,12 +51,6 @@ to newer interfaces.
 SPI marked `@_spi(Experimental)` should be assumed to be unstable. It may be
 modified or removed at any time.
 
-SPI marked `@_spi(ForSwiftTestingOnly)` is unstable and subject to change at any
-time.
-
-> [!WARNING]
-> Never use symbols marked `@_spi(ForSwiftTestingOnly)`.
-
 ## API and ABI stability
 
 When Swift Testing reaches its 1.0 release, API changes will follow the same

Sources/Overlays/_Testing_CoreGraphics/Attachments/Attachment+AttachableAsCGImage.swift

@@ -9,7 +9,7 @@
 //
 
 #if SWT_TARGET_OS_APPLE && canImport(CoreGraphics)
-@_spi(ForSwiftTestingOnly) @_spi(Experimental) public import Testing
+@_spi(Experimental) public import Testing
 
 public import UniformTypeIdentifiers
 

Sources/Overlays/_Testing_CoreGraphics/Attachments/ImageAttachmentError.swift

@@ -10,8 +10,7 @@
 
 #if SWT_TARGET_OS_APPLE && canImport(CoreGraphics)
 /// A type representing an error that can occur when attaching an image.
-@_spi(ForSwiftTestingOnly)
-public enum ImageAttachmentError: Error, CustomStringConvertible {
+package enum ImageAttachmentError: Error, CustomStringConvertible {
   /// The image could not be converted to an instance of `CGImage`.
   case couldNotCreateCGImage
 

Sources/Overlays/_Testing_Foundation/Attachments/Attachment+URL.swift

@@ -9,7 +9,7 @@
 //
 
 #if canImport(Foundation)
-@_spi(Experimental) @_spi(ForSwiftTestingOnly) public import Testing
+@_spi(Experimental) public import Testing
 public import Foundation
 
 #if !SWT_NO_PROCESS_SPAWNING && os(Windows)

Sources/Testing/Attachments/Attachment.swift

@@ -37,8 +37,7 @@ public struct Attachment<AttachableValue>: ~Copyable where AttachableValue: Atta
   public var fileSystemPath: String?
 
   /// The default preferred name to use if the developer does not supply one.
-  @_spi(ForSwiftTestingOnly)
-  public static var defaultPreferredName: String {
+  package static var defaultPreferredName: String {
     "untitled"
   }
 

Tests/TestingTests/AttachmentTests.swift

@@ -16,7 +16,7 @@ import Foundation
 #endif
 #if canImport(CoreGraphics)
 import CoreGraphics
-@_spi(Experimental) @_spi(ForSwiftTestingOnly) import _Testing_CoreGraphics
+@_spi(Experimental) import _Testing_CoreGraphics
 #endif
 #if canImport(UniformTypeIdentifiers)
 import UniformTypeIdentifiers

cmake/modules/shared/CompilerSettings.cmake

@@ -8,6 +8,8 @@
 
 # Settings intended to be applied to every Swift target in this project.
 # Analogous to project-level build settings in an Xcode project.
+add_compile_options(
+  "SHELL:$<$<COMPILE_LANGUAGE:Swift>:-package-name org.swift.testing>")
 add_compile_options(
   "SHELL:$<$<COMPILE_LANGUAGE:Swift>:-Xfrontend -require-explicit-sendable>")
 add_compile_options(