Drop Component.isSpecialDirectory

This API is fully redundant with .kind == .regular and could be a source of
confusion and namespace pollution. Drop it for now, and we can add something
equivalent in the future when we have more component-analysis APIs.

Author: milseman

Sources/System/FilePath/FilePathComponents.swift

@@ -94,9 +94,6 @@ extension FilePath.Component {
     if _path._isParentDirectory(_range) { return .parentDirectory }
     return .regular
   }
-
-  /// Whether this component is either special directory `.` or `..`.
-  public var isSpecialDirectory: Bool { _path._isSpecialDirectory(_range) }
 }
 
 extension FilePath.Root {
@@ -207,7 +204,7 @@ extension FilePath: _PlatformStringable {
 extension FilePath.Component {
   // The index of the `.` denoting an extension
   internal func _extensionIndex() -> SystemString.Index? {
-    guard !isSpecialDirectory,
+    guard kind == .regular,
           let idx = _slice.lastIndex(of: .dot),
           idx != _slice.startIndex
     else { return nil }

Sources/System/FilePath/FilePathSyntax.swift

@@ -325,7 +325,7 @@ extension FilePath {
     get { lastComponent?.extension }
     set {
       defer { _invariantCheck() }
-      guard let base = lastComponent, !base.isSpecialDirectory else { return }
+      guard let base = lastComponent, base.kind == .regular else { return }
 
       let suffix: SystemString
       if let ext = newValue {
@@ -373,7 +373,7 @@ extension FilePath {
     // `\..\foo\bar` should not.
     components.drop(
       while: { root == nil && $0.kind == .parentDirectory }
-    ).allSatisfy { !$0.isSpecialDirectory }
+    ).allSatisfy { $0.kind == .regular }
   }
 
   /// Collapse `.` and `..` components lexically (i.e. without following

Tests/SystemTests/FilePathTests/FilePathSyntaxTest.swift

@@ -916,14 +916,12 @@ final class FilePathSyntaxTest: XCTestCase {
       path.append(".")
       expect("/.")
       XCTAssert(path.components.last!.kind == .currentDirectory)
-      XCTAssert(path.components.last!.isSpecialDirectory)
       path.lexicallyNormalize()
       expect("/")
 
       path.append("..")
       expect("/..")
       XCTAssert(path.components.last!.kind == .parentDirectory)
-      XCTAssert(path.components.last!.isSpecialDirectory)
       path.lexicallyNormalize()
       expect("/")