Align/improve CMake/Conan packaging

[martinmoene]

See span-lite PR 37.

1. Install config where CMake find_package() can find it.
2. Make CMake install architecture-independent.
3. Improve conan/cmake interoperation.

---

Plan 1. Install config where CMake find_package() can find it.
In ./CMakeLists.txt, replace
set( package_folder "${package_nspace}" ) with
set( package_folder "${package_name}" ).

Progress 1:

• X - lest
• gsl-lite
• any-lite
• byte-lite
• expected-lite
• X - invoke-lite
• observer-ptr-lite
• X - optional-fun-lite
• optional-lite
• ring-span-lite
• X - scope-guard-lite (no code yet)
• span-lite (inspired this issue)
• status-value-lite
• string-view-lite
• value-ptr-lite
• variant-lite

---

Plan 2. Make CMake install arch independent:

• Add ./cmake/{project}-config-version.cmake.in (see span-lite's file)
• Change CMakeLists.txt: replace write_basic_package_version_file() with:

configure_file(
   "${CMAKE_CURRENT_SOURCE_DIR}/cmake/${package_name}-config-version.cmake.in"
   "${CMAKE_CURRENT_BINARY_DIR}/${package_config_version}" @ONLY
)

Progress 2:

• X - lest
• gsl-lite
• any-lite
• byte-lite
• expected-lite
• X - invoke-lite
• observer-ptr-lite
• X - optional-fun-lite
• optional-lite
• ring-span-lite
• X - scope-guard-lite (no code yet)
• span-lite (inspired this issue)
• status-value-lite
• string-view-lite
• value-ptr-lite
• variant-lite

---

Plan 3. Improve conan/cmake interoperation.
Change conanfile.py:

• Add "CMakeLists.txt", "cmake/*",
exports_sources = "include/nonstd/*", "CMakeLists.txt", "cmake/*", "LICENSE.txt"

• Change def package(self) to:

    def package(self):
        """Run CMake install"""
        cmake = CMake(self)
        cmake.definitions["{PROJECT}_OPT_BUILD_TESTS"] = "OFF"
        cmake.definitions["{PROJECT}_OPT_BUILD_EXAMPLES"] = "OFF"
        cmake.configure()
        cmake.install()

Progress 3:

• X - lest
• gsl-lite
• any-lite
• byte-lite
• expected-lite
• X - invoke-lite
• observer-ptr-lite
• X - optional-fun-lite
• optional-lite
• ring-span-lite
• X - scope-guard-lite (no code yet)
• span-lite (inspired this issue)
• status-value-lite
• string-view-lite
• value-ptr-lite
• variant-lite