Why are the python reasons for having our code nested in a directory
called ``swift_build_support.swift_build_support/`` instead of simply
``swift_build_support/``? Is that what we really want here?
Not all machines may have all platforms' SDKs installed.
xcodebuild already has a command to print the versions of all
SDKs installed. For convenience, also provide a function that
returns the identifiers for all installed SDKs.
rdar://problem/25618455
Rather than printing `xcodebuild` version information in the
build-script-impl shellscript, do so in the Python build-script. A small step
towards achieving SR-237.
