go: vendor the oapi-codegen cmd

See the comment in tools.go, I cannot fully explain what's happening here.
Somehow, Go 1.14 wants to use the vendored version of oapi-codegen but
without this file, oapi-codegen isn't vendored so the generation fails.

Signed-off-by: Ondřej Budai <ondrej@budai.cz>

internal/tools.go

@@ -0,0 +1,15 @@
+// +build tools
+
+// This file is here to just explicitly tell `go mod vendor` that we depend
+// on oapi-codegen. Without this file, `go generate ./...` in Go >= 1.14 gets
+// confused because oapi-codegen is not being vendored.
+//
+// This is apparently the conventional way, see:
+// https://stackoverflow.com/questions/52428230/how-do-go-modules-work-with-installable-commands
+// https://github.com/golang/go/issues/29516
+
+package main
+
+import (
+	_ "github.com/deepmap/oapi-codegen/cmd/oapi-codegen"
+)