Add Windows/Cygwin depexts for tools
Tools (versus libraries) have a single package on Cygwin which is used
by both the Cygwin build of opam and also by the native Windows builds
of opam.
Cygwin build: os = "cygwin" & os-distribution = "cygwin"
Native Windows: os = "win32" & os-dstribution = "cygwin"
(or os-distribution = "msys2")
Hence os-distribution = "cygwin" is a good filter for these shared
depexts.