Create a special Xcode selector for macOS

[cmcgee1024]

Add the feature of a special selector "xcode" that can be used like the other swiftly
selectors for some operations, such as swiftly use, and swiftly list, but has no
real effect on other operations.

With this feature, you can swiftly use xcode and then when you run the proxies for
items like swift, clang, and other common toolchain binaries they will be run on the
Xcode toolchain.

swift --version  => /Library/Developer/Toolchains/swift-xyz/usr/bin/swift --version
clang foo.c => /Library/Developer/Toolchains/swift-xyz/usr/bin/swift clang foo.c

When xcode is selected then swiftly run will run with the currently selected xcode
toolchain (or command-line tools) reported by xcrun -f as the toolchain on the path.

When you run swiftly list there is a special entry for "xcode" that is decorated in a
similar way to indicate whether it is the (global) default, and/or in-use toolchain as a
result of the selection mechanism.

TODO:

• Scan the toolchain on use and create necessary proxies

[cmcgee1024]

@swift-ci test macOS