Fix CPMAddPackage when FetchContent_MakeAvailable already added package (#288)

Add fetchcontent_dependency unit test:
* This test should highlight the fact that cpm_add_subdirectory is always called, even when
  cpm_fetch_package isn't populating the dependency
* NO_CACHE YES highlight a bug introduced in 32b063eba5 where
  cpm_fetch_package was checking undefined ${lower_case_name}_POPULATED variable

https://github.com/cpm-cmake/CPM.cmake/issues/287

test/unit/fetchcontent_dependency/dependency/CMakeLists.txt

@@ -0,0 +1,15 @@
+cmake_minimum_required(VERSION 3.14 FATAL_ERROR)
+
+project(CPMTest_Dependency)
+
+# ---- Dependencies ----
+
+include(FetchContent)
+
+FetchContent_Declare(
+  Fibonacci
+  GIT_REPOSITORY https://github.com/cpm-cmake/testpack-fibonacci.git
+  GIT_TAG v2.0
+)
+
+FetchContent_MakeAvailable(Fibonacci)