Fix DrugChemical conflation typing

[gaurav]

We previously used a randomly chosen identifier from each DrugChemical conflation to choose the Biolink type for the entire conflation, which would also determine the order of prefixes within the conflation. This lead to issues where we used an RXCUI to determine that a conflation should be considered a biolink:Drug, when really biolink:SmallMolecule would be a better type. Instead, this PR replaces that approach with a preferred-type approach.

Also replaces COMPLEX_CHEMICAL_MIXTURE with COMPLEX_MOLECULAR_MIXTURE, which is what Biolink calls it now.

Closes #264. Should be merged after PR #227.

[cbizon]

This is admittedly not exactly the envisioned use case but the idea of these consts was so that when a 1:1 change occurred here you could just change the string and save yourself many edits throughout the code.

[gaurav]

Yes, I know! But I took advantage of the centralized constants to bulk-rename one that had fallen out of sync with the corresponding Biolink type. If it had been a large change I would have proposed it in its own PR, but since it was only used in a handful of places and was related to the overall PR changes, I decided to incorporate it in here. For future changes, I'll update the string first and only update the constant name once it's pretty clear Biolink isn't going to change it again any time soon.