[CMAKE] Allow building secp256k1 as a standalone project

Summary:
This updates the module path to match the directory structure from
https://github.com/Bitcoin-ABC/secp256k1.

Depends on D4846.

Test Plan:
Build as part of the bitcoin-abc project and standalone from the github
repo.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D4844