countPlugins returning wrong number

[Rahe]

Description

Hello,

The method \Roots\Bedrock\Autoloader::countPlugins is returning a wrong number of mu-plugins if the mu-plugin isn't one.
Because the way of WordPress is counting mu-plugin is not the same as yours, the number can be wrong in the case of the folder on mu-plugins doesn't contains a php file with WordPress headers.

Steps to reproduce

1. Add humanmade/mercator dependency
2. Follow the installation process https://github.com/humanmade/Mercator#installation
3. Load WordPres 2 times
4. The updateCache method is called on every page

Expected behavior:
That only the real mu-plugins loaded by WordPress are effectively counted with get_plugins function.

Actual behavior:
The glob only counts the folders.

Reproduces how often:
100%

Versions

1.0.1, but the code haven't changed since.

Maybe strip away the mu-plugins without header to have the right count ?