Profiling exposed that we were spending significant amounts of
time recomputing dependency information in the DependencyStructure class.
Turns out that this was O(N^2) w/r/t to the number of solids during a
GraphQL query that gets information about the whole graph.
Built some indexes to fix this
With this change and https://github.com/dagster-io/dagster/issues/1967 our loading
time for the config editr for large schema is now 1-2 seconds.