add cmd/input/list to scan a plan for inputs

Signed-off-by: Tony Worm <tony@hofstadter.io>

tests/cli/input/scan/main.cue

@@ -0,0 +1,50 @@
+package main
+
+foo: string
+
+name:    string | *"world"
+message: "Hello, \(name)!"
+
+optional?: string
+
+missing: [string]: string
+
+bar: {
+	a:  string
+	#c: string
+	b:  int @dagger(computed)
+}
+
+// may be missing
+#inputs: {
+	hello:   string
+	missing: *"" | string
+}
+
+// substitute
+let A = string
+let B = bar.a
+
+//let Ba = bar.a
+//let Bb = bar.b
+let D = "hello"
+
+refd: {
+	a: string
+	b: {
+		ref1: a
+		ref2: A
+		aa:   B
+		bb:   D
+	}
+	#c: C: string
+}
+
+#fld1: string
+
+exec: {
+	cmd: string
+	#up: [{foo: string}]
+}
+
+list: [...string]