Make TerminalProgress a library. (#1093)

- Closes #1092.
- Allows 3rd party CLI plugins to use the progress indicator.

Package.swift

@@ -43,6 +43,7 @@ let package = Package(
         .library(name: "ContainerVersion", targets: ["ContainerVersion"]),
         .library(name: "ContainerXPC", targets: ["ContainerXPC"]),
         .library(name: "SocketForwarder", targets: ["SocketForwarder"]),
+        .library(name: "TerminalProgress", targets: ["TerminalProgress"]),
     ],
     dependencies: [
         .package(url: "https://github.com/apple/swift-log.git", from: "1.0.0"),