Patch not applied if metapackage

[ghost]

If a package is of type metapackage (cf https://getcomposer.org/doc/04-schema.md) and the unix patch method is used in vendor/cweagans/composer-patches/src/Patches.php, it fails with: 'Could not apply patch! Skipping. The error was: Cannot apply patch...'

This is because $install_path obtained in postInstall() is null, so in getAndApplyPatch() the -d arg to patch command is empty and causes the patch command to fail.

The attempt to write PATCHES.txt also fails as tries to write to root directory.

$install_path could be set to '.', which allows patch to complete - (though doesn't resolve how PATCHES.txt is used later).

I'm not sure why a number of the Drupal packages are set to metapackage in composer.lock, but they are sub-modules of a large overall module. Ideally I guess these should be of type: drupal-module

[ghost]

It turns out that using the main module as the key gets the patch applied to the sub-module.
Suggest a special error message if $install_path is empty rather than the generic 'Could not apply patch' when the patch command fails.

[cweagans]

Not sure if this is something that can be fixed. metapackage implies that there is no path that the package actually exists at, so it would be difficult for a patcher to apply a patch at that path. :)

I've made a note to explicitly avoid trying to patch metapackages in the plugin + add docs to explain why.