Use .buildCommand instead of .prebuildCommand

[maiyama18]

Now this library uses .prebuildCommand, so the plugin is executed on every build, which is unnecessary. It would be better if the plugin only run when needed by replacing .prebuildCommand by .buildCommand and specify input and output files properly.

https://github.com/maiyama18/LicensesPlugin/blob/7130ca28d84377fcbf830b68757fdcdb977abb20/Plugins/LicensesPlugin/LicensesPlugin.swift#L50-L55p