Resolves files owned by an OTP application's priv directory.
priv
Plugins can keep Rust and UI source files in their own package and identify the common root with an {app, root} tuple.
{app, root}
@type source() :: atom() | {atom(), String.t()}
@spec path(source(), String.t()) :: String.t()
@spec read!(source(), String.t()) :: String.t()