internal/abi: add a dummy Go file for go mod vendor

Before this change, the package `internal/abi` was ignored by `go mod vendor`
as there was no Go file there. Then building purego failed as `internal/abi`
didn't exist under the `vendor` directory.

This change fixes this issue by adding a dummy Go file to `internal/abi`
and adding an explicit import to it, but with a `dummy` build tag.

I referred a change goccy/go-graphviz#37 for this
hack.

Updates #31

dummy.go

@@ -0,0 +1,15 @@
+// SPDX-License-Identifier: Apache-2.0
+// SPDX-FileCopyrightText: 2022 The Ebitengine Authors
+
+//go:build dummy
+// +build dummy
+
+// This file exists purely to prevent the Go toolchain from stripping
+// away the c source directories and files when `go mod vendor` is used
+// to populate a `vendor/` directory of a project depending on this package.
+
+package purego
+
+import (
+	_ "github.com/ebitengine/purego/internal/abi"
+)

internal/abi/dummy.go

@@ -0,0 +1,8 @@
+// SPDX-License-Identifier: Apache-2.0
+// SPDX-FileCopyrightText: 2022 The Ebitengine Authors
+
+//go:build dummy
+// +build dummy
+
+// Package abi is a dummy package that prevents go tooling from stripping the C dependencies.
+package abi