System-installed liblzf/imgui not found

**Describe the bug**
A clear and concise description of what the bug is.

I'm trying to write a [Spack](https://spack.io) package for Open3D. In Spack, we try to avoid downloads during installation and instead use other Spack packages as external system-installed dependencies instead of 3rdparty vendored libraries. When I try to build with `-DUSE_SYSTEM_LIBLZF` and `-DUSE_SYSTEM_IMGUI`, CMake fails to find liblzf/imgui.

**To Reproduce**
Steps to reproduce the behavior:

```console
$ mkdir build
$ cd build
$ cmake .. -DUSE_SYSTEM_LIBLZF -DUSE_SYSTEM_IMGUI
...
CMake Warning at 3rdparty/find_dependencies.cmake:520 (find_package):
By not providing "Findliblzf.cmake" in CMAKE_MODULE_PATH this project has
asked CMake to find a package configuration file provided by "liblzf", but
CMake did not find one.

Could not find a package configuration file provided by "liblzf" with any
of the following names:

liblzfConfig.cmake
liblzf-config.cmake

Add the installation prefix of "liblzf" to CMAKE_PREFIX_PATH or set
"liblzf_DIR" to a directory containing one of the above files. If "liblzf"
provides a separate development package or SDK, be sure it has been
installed.
Call Stack (most recent call first):
CMakeLists.txt:613 (include)

-- Unable to find installed third-party library liblzf
-- Building library 3rdparty_lzf from source
...
CMake Warning at 3rdparty/find_dependencies.cmake:867 (find_package):
By not providing "FindImGui.cmake" in CMAKE_MODULE_PATH this project has
asked CMake to find a package configuration file provided by "ImGui", but
CMake did not find one.

Could not find a package configuration file provided by "ImGui" with any of
the following names:

ImGuiConfig.cmake
imgui-config.cmake

Add the installation prefix of "ImGui" to CMAKE_PREFIX_PATH or set
"ImGui_DIR" to a directory containing one of the above files. If "ImGui"
provides a separate development package or SDK, be sure it has been
installed.
Call Stack (most recent call first):
CMakeLists.txt:613 (include)

-- Unable to find installed third-party library ImGui
-- Building library 3rdparty_imgui from source
...
CMake Error at 3rdparty/find_dependencies.cmake:83 (add_library):
Cannot find source file:

/var/folders/21/hwq39zyj4g36x6zjfyl5l8080000gn/T/Adam/spack-stage/spack-stage-open3d-0.13.0-ue4nzmmyq3acgp2w5oawbreyrm6teax3/spack-src/3rdparty/imgui/imgui_demo.cpp

Tried extensions .c .C .c++ .cc .cpp .cxx .cu .mpp .m .M .mm .ixx .cppm .h
.hh .h++ .hm .hpp .hxx .in .txx .f .F .for .f77 .f90 .f95 .f03 .hip .ispc
Call Stack (most recent call first):
3rdparty/find_dependencies.cmake:880 (build_3rdparty_library)
CMakeLists.txt:613 (include)

CMake Error at 3rdparty/find_dependencies.cmake:83 (add_library):
No SOURCES given to target: 3rdparty_imgui
Call Stack (most recent call first):
3rdparty/find_dependencies.cmake:880 (build_3rdparty_library)
CMakeLists.txt:613 (include)

CMake Generate step failed. Build files cannot be regenerated correctly.
```

**Expected behavior**
A clear and concise description of what you expected to happen.

I would expect these dependencies to be found in my `CMAKE_PREFIX_PATH`.

**Environment (please complete the following information):**

- Operating system: macOS 10.15.7
- Python version: Python 3.8.12
- Open3D version: 0.13.0
- Is this remote workstation?: no
- How did you install Open3D?: build from source
- Compiler version (if built from source): Apple Clang 12.0.0

**Additional context**
Add any other context about the problem here.

Useful build logs:

* [build log](https://github.com/isl-org/Open3D/files/7584268/spack-build-out.txt)
* [build env](https://github.com/isl-org/Open3D/files/7584269/spack-build-env-mods.txt)