FMT_MODULE defaults on for subproject consumers and fails when clang-scan-deps is unavailable

When fmt is added as a subdirectory in a C++20 project using CMake 3.28+,
fmt may default `FMT_MODULE` to ON and add/build `fmt::fmt-module`, even when
the parent project only consumes `fmt::fmt-header-only` or `fmt::fmt`.

In Clang environments where `clang++` is available but `clang-scan-deps` is not
found by CMake, the build generates a command like:

CMAKE_CXX_COMPILER_CLANG_SCAN_DEPS-NOTFOUND -format=p1689 -- clang++ ...

and fails with:

/bin/sh: 1: CMAKE_CXX_COMPILER_CLANG_SCAN_DEPS-NOTFOUND: not found

Expected:
fmt should not build the C++20 module target by default for add_subdirectory
consumers that did not request modules, or the Clang native CMake modules path
should require `CMAKE_CXX_COMPILER_CLANG_SCAN_DEPS` to be found.

Workaround:
set(FMT_MODULE OFF CACHE BOOL "Build fmt module library." FORCE)
before add_subdirectory(fmt).

Caused issues for us when trying to upgrade fmtlib in PyTorch which uses CPP20, but not modules.
The clang containers did not install clang-scan-deps and therefore the compile runs crashed.

From PR: https://github.com/pytorch/pytorch/pull/187691

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