Discovers platform root when scanning single file

[gwillem]

Because corediff needs the relative path (to the platform root), it should check for each scanpath what the platform root should be.

Could add this

    cmsPaths = map[string]string{
        "/app/etc/local.xml":           "magento1",
        "/app/etc/env.php":             "magento2",
        "/wp-config.php":               "wordpress",
        "/lib/internal/Magento":        "magento2",
        "/app/design/frontend/Magento": "magento1",
    }