feat(tvix/store): add import::import_path

This imports the contents at a given Path into the tvix store.

It doesn't register the contents at a Path in the store itself, that's up
to the PathInfoService.

Change-Id: I2c493532d65b90f199ddb7dfc90249f5c2957dee
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8159
Reviewed-by: raitobezarius <tvl@lahfa.xyz>
Tested-by: BuildkiteCI

tvix/store/src/tests/mod.rs

@@ -1,3 +1,4 @@
 pub mod fixtures;
+mod import;
 mod nar_renderer;
 pub mod utils;