Backtracing: Use internal import in swift-backtracing.

The `swift-backtracing` module is an executable rather than a library so it
should have no reason to use the deprecated `@_implementationOnly import` over
`internal import`.

Author: tshortli

stdlib/public/libexec/swift-backtrace/TargetLinux.swift

@@ -29,7 +29,7 @@ import _Backtracing
 @_spi(MemoryReaders) import _Backtracing
 @_spi(Utils) import _Backtracing
 
-@_implementationOnly import Runtime
+internal import Runtime
 
 enum SomeBacktrace {
   case raw(Backtrace)

stdlib/public/libexec/swift-backtrace/TargetMacOS.swift

@@ -25,8 +25,8 @@ import _Backtracing
 @_spi(Contexts) import _Backtracing
 @_spi(MemoryReaders) import _Backtracing
 
-@_implementationOnly import Runtime
-@_implementationOnly import OS.Darwin
+internal import Runtime
+internal import OS.Darwin
 
 #if arch(x86_64)
 typealias MContext = darwin_x86_64_mcontext