create FindXXX.cmake modules for added CPM packages (#112)

* create FindXXX.cmake modules for added CPM packages

* don't use list(PREPEND) as it unsupported on older CMake versions

* use CMAKE_BINARY_DIR

* ensure CPM.cmake is initialized only once

* import CPM.cmake in modules

test/unit/test_project/.gitignore

@@ -0,0 +1 @@
+/CMakeLists.txt

test/unit/test_project/CMakeLists.txt.in

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+cmake_minimum_required(VERSION 3.14 FATAL_ERROR)
+
+project(CPMExampleCatch2)
+
+# ---- Options ----
+
+option(ENABLE_TEST_COVERAGE "Enable test coverage" OFF)
+
+# ---- Dependencies ----
+
+include(@CPM_PATH@/CPM.cmake)
+
+CPMAddPackage(
+  NAME @TEST_DEPENDENCY_NAME@
+  SOURCE_DIR ${CMAKE_CURRENT_LIST_DIR}/dependency
+)
+
+# ---- check if generated modules override find_package ----
+
+if (@test_check_find_package@)
+  find_package(@TEST_DEPENDENCY_NAME@ REQUIRED)
+endif()
+
+# ---- Call dependency method to validate correct addition of directory ----
+
+dependency_function()

test/unit/test_project/dependency/CMakeLists.txt

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+
+function(dependency_function)
+  message("called external method")
+endfunction()