Fix redundant access-level modifiers.

Sources/Basic/CollectionAlgorithms.swift

@@ -29,7 +29,7 @@ extension BidirectionalCollection where Iterator.Element : Comparable {
     }
 }
 
-public extension Sequence where Iterator.Element: Hashable {
+extension Sequence where Iterator.Element: Hashable {
 
     /// Finds duplicates in given sequence of Hashables.
     /// - Returns: duplicated elements in the invoking sequence.

Sources/Basic/StringConversions.swift

@@ -35,7 +35,7 @@ private func inShellWhitelist(_ codeUnit: UInt8) -> Bool {
     }
 }
 
-public extension String {
+extension String {
 
     /// Creates a shell escaped string. If the string does not need escaping, returns the original string.
     /// Otherwise escapes using single quotes. For example:

Sources/Commands/Destination.swift

@@ -160,7 +160,7 @@ public struct Destination {
   #endif
 }
 
-public extension Destination {
+extension Destination {
 
     /// Load a Destination description from a JSON representation from disk.
     public init(fromFile path: AbsolutePath, fileSystem: FileSystem = localFileSystem) throws {

Sources/Workspace/ToolsVersionWriter.swift

@@ -39,7 +39,7 @@ public func writeToolsVersion(at path: AbsolutePath, version: ToolsVersion, fs:
     try fs.writeFileContents(file, bytes: stream.bytes)
 }
 
-public extension ToolsVersion {
+extension ToolsVersion {
 
     /// Returns the tools version with zeroed patch number.
     public var zeroedPatch: ToolsVersion {