Warning for name="all"

Copying a comment from somewhere else:

> Are there actually targets named `all`? I thought that was reserved. This should be a strong ban on 'reserved' target names `all`, `__pkg__`, `__subpackages__`... any others? Build Doctor could start flagging these and filing bugs.

We could have a warning for this.

Start by locating Build Doctor’s target-validation entry point; the issue names no file or test. Confirm which names are reserved and define the warning behavior, with tests covering the agreed list. Done means reserved target names such as all, __pkg__, and __subpackages__ are flagged.

Written by the indexing model from the issue text.