feat(tvix/glue): handle regular file at builtins.path import

If builtins.path is passed a regular file, no filtering is applied.
We use the just-introduced file_type function in the EvalIO trait for
that.

This means, we don't need to pass through filtered_ingest, and can
assemble the FileNode directly in that specific match case.

This also means, we can explicitly calculate the sha256 flat digest,
and avoid having to pipe through the file contents again (via
blob_to_sha256_hash) to construct the sha256 digest.

Change-Id: I500b19dd9e4b7cc897d88b44547e7851559e5a4e
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11872
Tested-by: BuildkiteCI
Reviewed-by: Ilan Joselevich <personal@ilanjoselevich.com>
Reviewed-by: Connor Brewster <cbrewster@hey.com>

tvix/docs/src/TODO.md

@@ -127,15 +127,6 @@ Some more fetcher-related builtins need work:
  - `fetchTree` (hairy, seems there's no proper spec and the URL syntax seems
    subject to change/underdocumented)
 
-### `builtins.path` roundtrip for flat
-`builtins.path` currently uses `filtered_ingest` also for the non-recursive
-case, then reads through the blob contents again to get the sha256.
-
-We should take care of assembling the root node on our own, and pipe the data
-through sha256 too (via `InspectReader`, see `glue/fetcher` for an example).
-
-This avoids some roundtrips, and is probably faster.
-
 ### Derivation -> Build
 While we have some support for `structuredAttrs` and `fetchClosure` (at least
 enough to calculate output hashes, aka produce identical ATerm), the code