Isolate tools dependencies in tools package (#293)

Use `./tools` to prevent callers of `go-winio` from needing to add tools
to their dependencies.

Similar to https://github.com/microsoft/hcsshim/pull/1840

Signed-off-by: Hamza El-Saawy <hamzaelsaawy@microsoft.com>

tools/tools.go

@@ -0,0 +1,13 @@
+//go:build tools
+
+// This package contains imports to various tools used (eg, via `//go:generate`) within this repo.
+//
+// Calls to `go run <cmd/import/path>` (or `//go:generate go run <cmd/import/path>`) for go executables
+// included here will use the version specified in `go.mod` and build the executable from vendored code.
+//
+// Based on golang [guidance].
+//
+// [guidance]: https://github.com/golang/go/wiki/Modules#how-can-i-track-tool-dependencies-for-a-module
+package tools
+
+import _ "golang.org/x/tools/cmd/stringer"