Rename "capstan" package as "core"

The "go build" command won't work if there's a "capstan" directory in
the top-level directory. Rename the package to "core" instead.

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
build: Make capstan version work

go install does not work well with the

  -ldflags "-X main.VERSION '$version'"

trick. Fix by using go build instead.

Signed-off-by: Asias He <asias@cloudius-systems.com>