Add support for cycle-tolerant "weak" Gets

[stuhood]

Problem

As covered in #10059: in order to support fine-grained dependency inference on real world graphs (which very frequently involve file-level dependency cycles), we'd like to very carefully introduce a form of cycle-tolerance.

Solution

Introduce the concept of "weak" Gets (exposed via await Get(.., weak=True)) which can return None if a cycle would be formed that would cause a @rule to depend on its own result. A weak Get is intended to be similar in practice to a weak reference, and correspondingly non-weak edges are referred to as "strong" edges.

A few changes were made to the Graph to support this, but the most fundamental was that we now allow cycles in the Graph, as long as they don't involve any running Nodes. As described in the docstring for graph::Graph, we record dependencies for two reasons: 1) invalidation, and 2) deadlock detection. Invalidation is not concerned by cycles, and a deadlock can only occur in running code and thus does not need to walk non-live edges.

Result

Tests demonstrate that @rules are able to successfully consume "weak-weak" cycles, where a graph like:

digraph {
  A -> B [label=weak, style=dashed];
  B -> A [label=weak, style=dashed];
}

... produces the set {A, B} when entered from either direction. Followup work will additionally allow for "strong-weak" cycles, which "mostly" work right now, but which currently have behavior that depends on where the cycle is entered. See #10229 and the ignored test in this change.

[stuhood]

The commits are mostly useful to review independently, but the "Add engine support" and "Allow cycles to exist in the graph" commits overlap a bit.