Migrate to internal imports.

As of the Swift 6 compiler, `@_implementationOnly import` is deprecated in
favor of `internal import` and as a result the use of `@_implementationOnly
import` in this project is generating a lot of diagnostic noise when building
the Swift standard library.

For Swift libraries with library evolution, `@_implementationOnly import` and
`internal import` are roughly functionally equivalent, aside from improved
diagnostics for `internal import`. For non-resilient libraries, the main
difference is that `internal import` does not actually hide a module dependency
from downstream clients because the layout of a type in a non-resilient library
may depend on types coming from an `internal import` (with
`@_implementationOnly import` the same situation would result in a silent
mis-compile, which is the reason that `@_implementationOnly import` is
deprecated). The `_RegexParser` module dependency does not need to be hidden
from clients since it is installed in standard locations in the SDK/toolchain.
Therefore this migration should be safe, regardless of library resilience mode.

Author: tshortli

Sources/RegexBenchmark/Benchmark.swift

@@ -1,5 +1,5 @@
 @_spi(RegexBenchmark) import _StringProcessing
-@_implementationOnly import _RegexParser
+internal import _RegexParser
 import Foundation
 
 protocol RegexBenchmark: Debug {

Sources/RegexBuilder/Anchor.swift

@@ -9,7 +9,7 @@
 //
 //===----------------------------------------------------------------------===//
 
-@_implementationOnly import _RegexParser
+internal import _RegexParser
 @_spi(RegexBuilder) import _StringProcessing
 
 /// A regex component that matches a specific condition at a particular position

Sources/RegexBuilder/CharacterClass.swift

@@ -9,7 +9,7 @@
 //
 //===----------------------------------------------------------------------===//
 
-@_implementationOnly import _RegexParser
+internal import _RegexParser
 @_spi(RegexBuilder) import _StringProcessing
 
 /// A class of characters that match in a regex.

Sources/RegexBuilder/DSL.swift

@@ -9,7 +9,7 @@
 //
 //===----------------------------------------------------------------------===//
 
-@_implementationOnly import _RegexParser
+internal import _RegexParser
 @_spi(RegexBuilder) import _StringProcessing
 
 @available(SwiftStdlib 5.7, *)

Sources/_StringProcessing/ByteCodeGen.swift

@@ -12,7 +12,7 @@
 @_spi(_Unicode)
 import Swift
 
-@_implementationOnly import _RegexParser
+internal import _RegexParser
 
 extension Compiler {
   struct ByteCodeGen {

Sources/_StringProcessing/Capture.swift

@@ -9,7 +9,7 @@
 //
 //===----------------------------------------------------------------------===//
 
-@_implementationOnly import _RegexParser
+internal import _RegexParser
 
 // TODO: Where should this live? Inside TypeConstruction?
 func constructExistentialOutputComponent(

Sources/_StringProcessing/Compiler.swift

@@ -9,7 +9,7 @@
 //
 //===----------------------------------------------------------------------===//
 
-@_implementationOnly import _RegexParser
+internal import _RegexParser
 
 class Compiler {
   let tree: DSLTree

Sources/_StringProcessing/ConsumerInterface.swift

@@ -9,7 +9,7 @@
 //
 //===----------------------------------------------------------------------===//
 
-@_implementationOnly import _RegexParser
+internal import _RegexParser
 
 extension Character {
   var _singleScalarAsciiValue: UInt8? {

Sources/_StringProcessing/Engine/InstPayload.swift

@@ -9,7 +9,7 @@
 //
 //===----------------------------------------------------------------------===//
 
-@_implementationOnly import _RegexParser
+internal import _RegexParser
 
 extension Instruction {
   /// An instruction's payload packs operands and destination

Sources/_StringProcessing/Engine/MEBuilder.swift

@@ -9,7 +9,7 @@
 //
 //===----------------------------------------------------------------------===//
 
-@_implementationOnly import _RegexParser // For errors
+internal import _RegexParser // For errors
 
 extension MEProgram {
   struct Builder {