reorganize main module to import from the utpm library

[ensko]

The main.rs and lib.rs declare the same modules (utils, commands, build through shadow_rs). This is a bit of a footgun, because the library and binary are two separate crates. Worst case, something like this can happen:

// lib.rs
mod x;

// main.rs
mod x;

fn main() {
    x::foo();  // modifies a static variable
    utpm::x::bar();  // reads that static variable, but it's actually a different one
}

I thus removed all modules declared in main.rs and replaced them with imports of utpm::.... If my intuition is correct, this should also improve compile times, since the utpm could should previously have been compiled twice. (Edit: CI runs indicate there is a time difference, although not as significant as halving compilation time would be.)