Detect circular references

A feature-flag file is lines of the exact form "KEY=value" (split on the first "="). Values may mention other keys with the syntax "${OTHER}". Your linter must reject files where following such mentions can loop forever. Treat each mention of an EXISTING key as an edge from the defining key to the mentioned key; mentions of undefined names are harmless and ignored, as is any malformed "${" without a closing "}". Return true when the resulting graph contains a cycle (a key mentioning itself counts), false otherwise.