Use an `ExactSizeIterator` for target dependency enumeration

Allows code to check `target.dependencies().len()`.

Change-Id: I44c0a616e06928764ffbf744ff3eb5a6fd2888c8
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/qg/+/126256
Commit-Queue: Auto-Submit <auto-submit@pigweed.google.com.iam.gserviceaccount.com>
Pigweed-Auto-Submit: Erik Gilling <konkers@google.com>
Reviewed-by: Alexei Frolov <frolv@google.com>