Add `customSourcePackagesPath` to build plugin

[briannadoubt]

When using xcodebuild from the CLI with a -clonedSourcePackagesDirPath flag, this plugin fails. By adding a custom path to the config file we can leverage this value, and set the path up in a custom way to match our CLI. Because of the way the plugin was already doing the possibleSourcePackagesPaths check we can just add this path there and it doesn't break local builds.