Use interfaces instead of namespaces in externs

[theseanl]

Use interfaces instead of namespaces in externs

[evmar]

Thanks for filing this separately, I can ask some people who know more than me about it.

[evmar]

Note: tsickle only generates a namespace per file for module d.ts, not for script (aka 'global') dts.

[blickly]

Can you include a code snippet to show what the current behavior is that you're proposing to move away from?

[theseanl]

Current behavior would be the following:

/** @externs */

/**********************************************************************/

/** @const */
var myNs1 = {};

/**
 * @constructor
 * @struct
 */
myNs1.X = function() {};

/** @type {string} */
msNs1.X.prototype.foo;

/**********************************************************************/

/** @const */
var myNs2 = {};

/**
 * @record
 * @struct
 */
myNs2.Y = function() {};

/** @type {string} */
myNs2.Y.prototype.bar = function() {};

/**********************************************************************/

// /**
//  * @const {(typeof myNs1|typeof myN2)}
//  */
// var myNs;