Detect URLs referring to subdirectories

[SomeoneSerge]

e.g. in monorepos: https://github.com/langchain-ai/langchain/blob/master/libs/langchain/pyproject.toml.

Expected behaviour

$ nix-init https://github.com/langchain-ai/langchain/
... parses the top-level pyproject.toml
$ nix-init https://github.com/langchain-ai/langchain/blob/master/libs/langchain/
... parses libs/langchain/pyproject.toml

OTOH, we could opt for something more explicit than the special treatment of blob/ref/... urls

[Kranzes]

Depends on #118