cue modules: move stdlib to pkg/alpha.dagger.io

In preparation for Europa, we will vendor multiple CUE modules:

- `pkg/alpha.dagger.io`: legacy non-europa packages
- `pkg/dagger.io`: core Europa packages
- `pkg/universe.dagger.io`: Europa universe

Signed-off-by: Andrea Luzzardi <aluzzardi@gmail.com>

pkg/alpha.dagger.io/os/file.cue

@@ -0,0 +1,43 @@
+// OS operations
+package os
+
+import (
+	"alpha.dagger.io/dagger"
+	"alpha.dagger.io/dagger/op"
+)
+
+// Built-in file implementation, using buildkit
+// A single file
+#File: {
+	from: dagger.#Artifact | *[op.#Mkdir & {dir: "/", path: "/"}]
+	path: string
+
+	// Optionally write data to the file
+	write: *null | {
+		data: string
+		// FIXME: append
+		// FIXME: create + mode
+	}
+
+	// The contents of the file
+	// If a write operation is specified, it is applied first.
+	contents: {
+		string
+
+		#up: [
+			op.#Load & {
+				"from": from
+			},
+			if write != null {
+				op.#WriteFile & {
+					dest:    path
+					content: write.data
+				}
+			},
+			op.#Export & {
+				source: path
+				format: "string"
+			},
+		]
+	}
+}