[plan] Plan.Definitions add omitempty (#171)

## Summary

I'm adding omitempty to Plan.Definitions.

I'm not adding it to Plan.DevPackages and Plan.RuntimePackages. Because
it explains to a reader that these are empty. These fields are so core
and commonly used that I think we want to make an exception for them.

## How was it tested?

compiles

Author: savil

planner/plansdk/plansdk.go

@@ -52,7 +52,7 @@ type Plan struct {
 	// Ex: python main.py
 	StartStage *Stage `json:"start_stage,omitempty"`
 
-	Definitions []string `cue:"[...string]" json:"definitions"`
+	Definitions []string `cue:"[...string]" json:"definitions,omitempty"`
 	// Errors from plan generation. This usually means
 	// the user application may not be buildable.
 	Errors []PlanError `json:"errors,omitempty"`

testdata/zig/zig-hello-world/plan.json

@@ -17,6 +17,5 @@
     "input_files": [
       "./zig-out/bin/"
     ]
-  },
-  "definitions": null
+  }
 }