CMake build is missing key features to be properly usable via FetchContent

[globberwops]

Describe the bug

The CMake build is missing key features to be properly usable via CMake's FetchContent module.

Describe how to reproduce the bug

Steps to reproduce the behavior:

1. In a downstream CMake project, do

include(FetchContent)
FetchContent_Declare(
  open-simulation-interface
  GIT_REPOSITORY https://github.com/OpenSimulationInterface/open-simulation-interface.git
  GIT_TAG v3.3.1
)
FetchContent_MakeAvailable(open-simulation-interface)

2. This will
   a. pollute your upstream project's cache with non-prefixed variables INSTALL_LIB_DIR, INSTALL_INCLUDE_DIR, and INSTALL_CMAKE_DIR
   b. not provide namespaced alias targets e.g. open-simulation-interface::static
   c. not install the export set with a namespace
   d. pollute the downstream project's build with an unnecessary, redundant warning:
   

   open-simulation-interface/CMakeLists.txt

   Line 186 in ab60959

   message(WARNING "Doxygen could not be found.")

Describe the expected behavior

1. prefix cache variables with the project name
2. provide namespaced alias targets usable from the build tree
3. provide a namespace for the export set
4. do not warn unnecessarily