cmake: move pkg-config names to Find modules

[vszakats]

Make the Find modules set and return their respective pkg-config
module name(s) to the CMake build process, which then adds those
to the Requires: list.

Before this patch, pkg-config module names were maintainted in two
separate places. After this patch, they are maintained in the Find
modules for dependencies that have one (most do).

Re-align existing modules with this change: msh3, mbedtls, rustls.
These modules return their pkg-config module name only when
detected via pkg-config.

Follow-up to d511ec8 #15573

---

• rebase onto cmake/FindLDAP: avoid empty 'Requires' item when omitting pkg-config module #15828

[testclutch]

Analysis of PR #15800 at 6eb6e718:

Test 1198 failed, which has NOT been flaky recently, so there could be a real issue in this PR.

Generated by Testclutch