[CMAKE] Improve FindSHLWAPI

Summary:
This makes the FindSHLWAPI consistent with the other find modules.
There are few benefits right now because the imported target cannot be
used due to cmake bugs
(https://gitlab.kitware.com/cmake/cmake/issues/16291,
https://gitlab.kitware.com/cmake/cmake/issues/19095) but makes it future
proof when cmake > 3.14.1 get enforced.

Test Plan:

ninja check

Run the Gitian builds.

I also tested using the imported target with cmake 3.14.1.

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D5453