Rename _MatchingEngine module to _RegexParser (#42081)

As the _MatchingEngine module no longer contains the matching engine, this patch renames this module to describe its role more accurately. Because this module primarily contains the AST and the regex parsing logic, I propose we rename it to "_RegexParser".

Also renames the ExperimentalRegex module in SwiftCompilerSources to _RegexParser for consistency. This would prevent errors if sources in _RegexParser used qualified lookup with the module name.

SwiftCompilerSources/Package.swift

@@ -11,7 +11,7 @@ let package = Package(
     .library(
       name: "Swift",
       type: .static,
-      targets: ["SIL", "Optimizer", "ExperimentalRegex"]),
+      targets: ["SIL", "Optimizer", "_RegexParser"]),
   ],
   dependencies: [
   ],
@@ -26,7 +26,7 @@ let package = Package(
           "-cross-module-optimization"
         ])]),
     .target(
-      name: "ExperimentalRegex",
+      name: "_RegexParser",
       dependencies: [],
       swiftSettings: [SwiftSetting.unsafeFlags([
           "-I", "../include/swift",
@@ -34,7 +34,7 @@ let package = Package(
         ])]),
     .target(
       name: "Optimizer",
-      dependencies: ["SIL", "ExperimentalRegex"],
+      dependencies: ["SIL", "_RegexParser"],
       swiftSettings: [SwiftSetting.unsafeFlags([
           "-I", "../include/swift",
           "-cross-module-optimization"