Allow a single installer path to be declared as a string

[tstoeckler]

Currently when you declare an installer path as a string versus an array, i.e. something like:

        "installer-paths": {
            "web/modules/{$name}": "type:drupal-module",
        }

you get an error:

in_array() expects parameter 2 to be array, string given
/home/tobias/Projekte/installers/src/Composer/Installers/BaseInstaller.php:130
/home/tobias/Projekte/installers/src/Composer/Installers/BaseInstaller.php:58
/home/tobias/Projekte/installers/src/Composer/Installers/Installer.php:158

I would like this to be possible. Not because I have a problem with the array structure in general, but because I would like to be able to configure the installer paths via the composer config command-line and that only allows 3 levels of nesting. I.e. I would like to be able to run

composer config extra.installer-paths.web/modules/{\$name} type:drupal-module

without that breaking the configuration.

Because a very common configuration is to have only a single path anyway, it seems like it is not a huge divergence from the status quo.

And it seemed easy enough to implement without adding much complexity. Also added a test and verified locally that it triggers the notice above without the fix.

Thoughts?

[effulgentsia]

It would definitely be great to be able to add installer paths via composer config.

If for some reason, this project's maintainers don't want the complexity of values that can be either strings or arrays, another option I suggested in composer/composer#8353 (comment) is to add a --json option to composer config, which would allow:
composer config --json extra.installer-paths.your/custom/path/{\$name}/ '["vendor:my_organization"]'

Personally, I'd be happy with either or both, or any other solution that allows installer paths to be added via composer config.

[Seldaek]

Seems reasonable enough, thanks