title: import with conditional argument in jsonnet
body: <p>I would like to have the ability to specify different arguments to the imported variable and change the behaviour of import. For example, I would like to have something like the following:</p>
  
  <pre><code>local foo = import "foo.libsonnet";
  {
    "foo1": foo["foo1"], 
    "foo2": foo["foo2"]
  }
  </code></pre>
  
  <p>foo.libsonnet: (something like the switch case in programming that has a default value) I know the following should not work. Just maybe pseudocode for what it could be used.</p>
  
  <pre><code>{
    "foo1": "bar1", // only if foo1 is passed
    "foo2": "bar2", // only if foo2 is passed
  
    "bar1" : "bar1_value",//default 
    "bar2" : "bar2_value" //default 
  }
  </code></pre>
  
  <p>output:</p>
  
  <pre><code>{
  "foo1":{
    "foo1": "bar1",
    "bar1": "bar1_value",
    "bar2" : "bar2_value"
    },
  "foo2":{
    "foo2": "bar2",
    "bar1": "bar1_value",
    "bar2" : "bar2_value"
    }
  }
  </code></pre>