Fix behavior of file rebasing when file starts with "file:./"

Relative files that are located in the same directory as the
package.json, e.g. `"file:./foo.tgz"`, were not being rebased correctly
by `rebaseFileReferences` due to a bad regular expression. The regex has
been fixed in this commit.

lib/packExternalModules.js

@@ -9,7 +9,7 @@ const isBuiltinModule = require('is-builtin-module');
 const Packagers = require('./packagers');
 
 function rebaseFileReferences(pathToPackageRoot, moduleVersion) {
-  if (/^(?:file:[^/]{2}|\.\/|\.\.\/)/.test(moduleVersion)) {
+  if (/^(file:[^/]|\.\/|\.\.\/)/.test(moduleVersion)) {
     const filePath = _.replace(moduleVersion, /^file:/, '');
     return _.replace(
       `${_.startsWith(moduleVersion, 'file:') ? 'file:' : ''}${pathToPackageRoot}/${filePath}`,
@@ -446,4 +446,3 @@ module.exports = {
     });
   }
 };
-