om init FLAKE_URL -o OUTPUT_DIR on existing OUTPUT_DIR

[shivaraj-bh]

The current behaviour is to fail if OUTPUT_DIR exists. But there can be cases where initialising templates in existing project folders can be useful, for example if one wants to initialise only the .nix files from https://github.com/srid/rust-nix-template in their existing project (The existing template only supports initialising entire rust-nix-template source, but there can be a filtered version for .nix files). The only way to do this now, on existing projects, is to manually copy-paste (and modify) .nix files from one of the examples.

Not relevant (low-level rust-nix-template detail):

There is also the problem that rust-nix-template’s nix modules will not just work-out-of-the-box on all rust projects. Perhaps the om.templates.nix-only (to be implemented conditional to the above proposal getting accepted) can support param to accept a list of crate names to initialise pre-crate crate.nix files for.

Note: This would still not guarantee to work-out-of-the-box because there can be non-rust library dependencies that crates can depend on; which must be manually added (for example: openssl in omnix).