Installer module should check for file-write permissions

Description of the need We have this problem on Pantheon with the installer module: If the site is not in SFTP mode, then the installer can't install.

We know that the installer can't install, so when we try to install something, we throw a nice error saying that there's a permissions issue.

Proposed solution Why don't we use this check as a dependency for the installer module? If you can't write modules to your filesystem, then you can't use this module.