Fix caching warnings when running under Embroider

Prior to this change, you would receive the following warnings during an Embroider build:

```
broccoli-babel-transpiler is opting out of caching due to a plugin that does not provide a caching strategy: `function(env) {
  let b = env.syntax.builders;
  let transform = (node) => {
    if ('sexpr' in node) {
      node = node.sexpr;
    }

    let testSelectorParams = [];
    let otherParams = [];

    node.params.forEach(function(param) {
      if (isTestSelectorParam(param)) {
        testSelectorParams.push(param);
      } else {
        otherParams.push(param);
      }
    });

    node.params = otherParams;

    testSelectorParams.forEach(function(param) {
      let pair = b.pair(param.original, b.boolean(true));
      node.hash.pairs.push(pair);
    });
  };

  return {
    name: 'TransformTestSelectorParamsToHashPairs',

    visitor: {
      MustacheStatement: transform,
      BlockStatement: transform,
    },
  };
}`
```

The reason for this warning is that Embroider is attempting to read the caching
information (baseDir and cacheKey) from the plugin itself (not the wrapper we
register). Some recent changes in Embroider have improved things a bit
(reducing the number of cases where this warning will occur), but fixing
all scenarios is quite difficult.

---

This PR works around the issue by placing the caching information on
both the registries wrapper object **and** the plugin itself (ensuring
that there is still a single source of truth).

This fixes the warning you get when running in Embroider builds.

index.js

@@ -112,8 +112,8 @@ module.exports = {
     return {
       name: 'strip-test-selectors',
       plugin: StripTestSelectorsTransform,
-      baseDir() { return __dirname; },
-      cacheKey() { return 'strip-test-selectors'; },
+      baseDir: StripTestSelectorsTransform.baseDir,
+      cacheKey: StripTestSelectorsTransform.cacheKey,
     };
   },
 
@@ -123,8 +123,8 @@ module.exports = {
     return {
       name: 'transform-test-selector-params-to-hash-pairs',
       plugin: TransformTestSelectorParamsToHashPairs,
-      baseDir() { return __dirname; },
-      cacheKey() { return 'transform-test-selector-params-to-hash-pairs'; },
+      baseDir: TransformTestSelectorParamsToHashPairs.baseDir,
+      cacheKey: TransformTestSelectorParamsToHashPairs.cacheKey,
     };
   },
 };

strip-test-selectors.js

@@ -29,3 +29,11 @@ module.exports = function() {
     }
   };
 };
+
+module.exports.baseDir = function() {
+  return __dirname;
+};
+
+module.exports.cacheKey = function() {
+  return 'strip-test-selectors';
+};

transform-test-selector-params-to-hash-pairs.js

@@ -44,3 +44,11 @@ module.exports = function(env) {
     },
   };
 };
+
+module.exports.baseDir = function() {
+  return __dirname;
+};
+
+module.exports.cacheKey = function() {
+  return 'transform-test-selector-params-to-hash-pairs';
+};