Add Support for LICENSE.txt

[jheidschwarz]

Please consider a logic like that to support .txt LICENSE files

let path = "\(packageCachePath)/\(pkgDir)/LICENSE"

let data: Data? = {
    if let data = fman.contents(atPath: path) {
        return data
    } else {
        return fman.contents(atPath: "\(path).txt")
    }
}()

guard let data else { return }

[MartinP7r]

I believe this was supported in

• support multiple license files #21