Mirror nix’s default.nix mechanism in path discovery

[Profpatsch]

I forgot to post my findings, but yes this is true! Nix doesn't emit /path/to/default.nix as the true file, it emits just '/path/to. A possible solution here is to append a /default.nix` when an imported directory is identified.

Originally posted by @grahamc in #21

We currently watch the whole directory if a user does import /path/to/directory. Nix implicitely assumes /path/to/directory/default.nix if used with import. We should mirror that, to prevent watching too many files.

[grahamc]

I think the solution here is to extend LogDatam to have an EvalSource variant:

lorri/src/builder.rs

Lines 128 to 131 in f93989b

	enum LogDatum {
	Source(PathBuf),
	Text(OsString),
	}

and then make use of the LogDatum::EvalSource here:

lorri/src/builder.rs

Lines 155 to 156 in f93989b

	if let Some(matches) = EVAL_FILE.captures(&linestr) {
	LogDatum::Source(PathBuf::from(&matches["source"]))

then when we're adding the list of paths at:

lorri/src/builder.rs

Line 101 in f93989b

match result {

match EvalSource, do an is_dir check, and push a default.nix if is_dir returns true.