builder: mirror nix’s default.nix mechanism

For a directory import like `import ./foo`, nix imports
`./foo/default.nix` but only prints `./foo` to its log output.
This commit reproduces that behaviour.

We were watching way too many files, e.g. `import <nixpkgs> {}` meant
we’d watch all of nixpkgs recursively. This should be solved now.

Fixes #58

src/builder.rs

@@ -103,9 +103,21 @@ fn instrumented_instantiation(
             .into_iter()
             .fold((vec![], vec![]), |(mut paths, mut log_lines), result| {
                 match result {
-                    LogDatum::CopiedSource(src)
-                    | LogDatum::NixSourceFile(src)
-                    | LogDatum::ReadFileOrDir(src) => {
+                    LogDatum::CopiedSource(src) | LogDatum::ReadFileOrDir(src) => {
+                        paths.push(src);
+                    }
+                    LogDatum::NixSourceFile(mut src) => {
+                        // We need to emulate nix’s `default.nix` mechanism here.
+                        // That is, if the user uses something like
+                        // `import ./foo`
+                        // and `foo` is a directory, nix will actually import
+                        // `./foo/default.nix`
+                        // but still print `./foo`.
+                        // Since this is the only time directories are printed,
+                        // we can just manually re-implement that behavior.
+                        if src.is_dir() {
+                            src.push("default.nix");
+                        }
                         paths.push(src);
                     }
                     LogDatum::Text(line) => log_lines.push(line),