Fix defaultRepos fallback does not use auth config
Fix defaultRepos fallback does not use auth config

When a full 'composer' cannot be constructed (because there is no
local composer.json and no global composer.json), some commands
(e.g. `show -a`) fall back to the default repositories from the
`$COMPOSER_HOME/config.json` file. Without this fix, any auth
configuration from `$COMPOSER_HOME/auth.json` is not used for
these repositories in such a fallback scenario.

Steps to reproduce:

* Configure a password-protected composer repository in
  `$COMPOSER_HOME/config.json`.
* Configure valid credentials for that repository in
  `$COMPOSER_HOME/auth.json`.
* Make sure there is no file `$COMPOSER_HOME/composer.json`.
* Ensure the current working directory has no `composer.json`.
* Run `composer show -a some/package`.

Expected: Information about `some/package` is shown without
needing to enter credentials.

Actual: A prompt "Authentication required" is shown for the
private repository. When running the same command in a dir
that has a `composer.json`, or when `$COMPOSER_HOME/composer.json`
exists, things work as expected.

staabm/composer,nicolas-grekas/composer,stefangr/composer,alcohol/composer,milleruk/composer,andrerom/composer,pierredup/composer,naderman/composer,slbmeh/composer,fabpot/composer,johnstevenson/composer,composer/composer,stof/composer,floverdevel/composer,localheinz/composer