[anneal][v2] Implement out-of-tree dependency chasing for expand command

Notes about largely unreviewed generated change:

- Last test assertion looks dubious. Should probably switch all LLBC
  test verifications to something that properly parses the LLBC and
  makes more precise assertions.

- Integration and structure requires more thought. It seems that only
  the scanner "knows" about this behaviour; is it being being plumbed
  through cmdline args and through run_charon?

- New test (and probably other tests) should have their setup procedures
  overhauled/simplified. In general, I would expect the test to create
  a fixture, and then be able to pass largely static data to
  `run_charon()`, but right now it's manually setting up locked roots,
  etc., etc.

gherrit-pr-id: Gxsjyiksx4pd6h5gzup7ensn4qtrxu6s3

Author: mdittmer

anneal/v2/Cargo.toml

@@ -4,6 +4,8 @@ members = ["."]
 [features]
 # Enables tests that assume a prebuilt exocrate archive.
 exocrate_tests = []
+# Enables tests that make network requests or download cargo dependencies.
+online_tests = []
 
 [package]
 name = "cargo-anneal"