FS caching improvements (#375)

* Instantiate default cache directory lazily, and only once

When using `cacheDirectory=true`, the `findCacheDir` module
is repeatedly called, and it does some synchronous filesystem
calls under the hood. However, since `findCacheDir` is more
or less a pure function, it's probably safe to call it only
once during process lifetime.

* Cache resolve-rc results

src/fs-cache.js

@@ -15,6 +15,8 @@ const os = require("os");
 const path = require("path");
 const zlib = require("zlib");
 
+let defaultCacheDirectory = null;  // Lazily instantiated when needed
+
 /**
  * Read the contents from the compressed file.
  *
@@ -161,13 +163,17 @@ const handleCache = function(directory, params, callback) {
  *
  *   });
  */
+
 module.exports = function(params, callback) {
   let directory;
 
   if (typeof params.directory === "string") {
     directory = params.directory;
   } else {
-    directory = findCacheDir({ name: "babel-loader" }) || os.tmpdir();
+    if (defaultCacheDirectory === null) {
+      defaultCacheDirectory = findCacheDir({ name: "babel-loader" }) || os.tmpdir();
+    }
+    directory = defaultCacheDirectory;
   }
 
   handleCache(directory, params, callback);

src/resolve-rc.js

@@ -10,6 +10,8 @@ const path = require("path");
 const exists = require("./utils/exists")({});
 const read = require("./utils/read")({});
 
+const cache = {};
+
 const find = function find(start, rel) {
   const file = path.join(start, rel);
 
@@ -27,6 +29,9 @@ const find = function find(start, rel) {
 
 module.exports = function(loc, rel) {
   rel = rel || ".babelrc";
-
-  return find(loc, rel);
+  const cacheKey = `${loc}/${rel}`;
+  if (!(cacheKey in cache)) {
+    cache[cacheKey] = find(loc, rel);
+  }
+  return cache[cacheKey];
 };