infrakit: Move the hyperkit instance plugin into the source directory

- The tools directory ideally should not contain source code
- Removes double vendoring of packagages
- Makes it easer to hook the build into the top-level Makefile

Eventually, the plugin should be moved to the infrakit repo.

Signed-off-by: Rolf Neugebauer <rolf.neugebauer@docker.com>

vendor/github.com/spf13/cobra/command_notwin.go

@@ -0,0 +1,5 @@
+// +build !windows
+
+package cobra
+
+var preExecHookFn func(*Command)