Remove Findfmt.cmake

[domin144]

Description

fmt provides cmake config files since at least version 7.0.0, which is the minimum version supported.
Fixed some variable names, so the provided config can be used.
The previous variable names were missing, if the library was installed in a custom location,
like in case of manual build or provided by conan.

Tests

Checklist:

• I have read the contribution guidelines.
• I have updated the documentation, if applicable.
• I have ensured that the change is tested somewhere in the testsuite
  (adding new test cases if necessary).
• If I added or modified a C++ API call, I have also amended the
  corresponding Python bindings (and if altering ImageBufAlgo functions, also
  exposed the new functionality as oiiotool options).
• My code follows the prevailing code style of this project. If I haven't
  already run clang-format before submitting, I definitely will look at the CI
  test that runs clang-format and fix anything that it highlights as being
  nonconforming.

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: domin144 / name: Dominik Wójt (7688693, a1ad652, b75f7c1, 6e011f0, 4746206, 528a0fa, 2813308, 8dbf427)

[domin144]

Conan still does not like this solution:

CMake Error at src/include/CMakeLists.txt:77 (file):
  file COPY cannot find
  "/home/domin144/.conan2/p/b/openi2b837708c2554/b/src/src/include/$<$<CONFIG:Release>:/home/domin144/.conan2/p/b/fmt8d14cec70a06c/p/include>/fmt/core.h":
  No such file or directory.

[domin144]

conan can be made happy with INTERNALIZE_FMT=OFF. Still, there may be other contexts, where INTERFACE_INCLUDE_DIRECTORIES contains generator expressions and INTERNALIZE_FMT is on.
I am open to comments on how can this be solved.

[domin144]

Not really proud of the changes, but it should work now.

[lgritz]

Sorry for the delay, I've been travelling (still am, but had a little time today to catch up here).

Conceptually, it's fine with me to get rid of Findfmt.cmake since our minimum fmt no longer requires it, and then cleaning up what remains.

Looks like the changes as they stand are failing most of the CI tests, though.

CMake Error at src/cmake/externalpackages.cmake:354 (add_library):
  add_library called with IMPORTED argument but no library type.

I'll merge once that gets cleaned up.

[domin144]

Another attempt. If this does not work, I will have to get back to this next week.

[domin144]

It took some tries, but it finally passes.

[lgritz]

LGTM, thanks for the fixes