refactor: Correct under-reported code coverage for Options/parsers

buildConfigDefinitions imported parsers from ../src/Options/parsers at
module scope. Loading it in a spec instruments the real src file a
second time (top level only), colliding with the sourcemap-remapped lib
coverage from parsers.spec.js and clobbering it down to ~2.5%. Move the
require into parseDefaultValue, its only consumer, so the module reports
its true 100%.

Author: dblythy

resources/buildConfigDefinitions.js

@@ -9,7 +9,6 @@
  * To rebuild the definitions file, run
  * `$ node resources/buildConfigDefinitions.js`
  */
-const parsers = require('../src/Options/parsers');
 
 /** The types of nested options. */
 const nestedOptionTypes = [
@@ -190,6 +189,8 @@ function mapperFor(elt, t) {
 }
 
 function parseDefaultValue(elt, value, t) {
+  /* istanbul ignore next: lazy require (not module scope) so specs don't double-instrument parsers.js; only reached by `npm run definitions` */
+  const parsers = require('../src/Options/parsers');
   let literalValue;
   if (t.isStringTypeAnnotation(elt)) {
     if (value == '""' || value == "''") {