Fix support for source subdirectories (#238)

* add support for source subdirectories

* style fixes

* remove debug log

* grammar fix in comment

test/unit/local_dependency/SubdirCMakeLists.txt.in

@@ -0,0 +1,17 @@
+cmake_minimum_required(VERSION 3.14 FATAL_ERROR)
+
+project(CPMTest)
+
+# ---- Dependencies ----
+
+include(@CPM_PATH@/CPM.cmake)
+
+CPMAddPackage(
+  NAME Dependency
+  SOURCE_DIR ${CMAKE_CURRENT_LIST_DIR}/dependency
+  SOURCE_SUBDIR inner
+)
+
+# ---- Call inner dependency method to validate correct addition of subdirectory ----
+
+inner_dependency_function()

test/unit/local_dependency/dependency/inner/CMakeLists.txt

@@ -0,0 +1,3 @@
+function(inner_dependency_function)
+  message("called inner method")
+endfunction()

test/unit/subdir.cmake

@@ -0,0 +1,17 @@
+include(CMakePackageConfigHelpers)
+include(${CPM_PATH}/testing.cmake)
+
+set(TEST_BUILD_DIR ${CMAKE_CURRENT_BINARY_DIR}/source_dir)
+
+configure_package_config_file(
+  "${CMAKE_CURRENT_LIST_DIR}/local_dependency/SubdirCMakeLists.txt.in"
+  "${CMAKE_CURRENT_LIST_DIR}/local_dependency/CMakeLists.txt"
+  INSTALL_DESTINATION ${CMAKE_CURRENT_BINARY_DIR}/junk
+)
+
+execute_process(
+  COMMAND ${CMAKE_COMMAND} "-H${CMAKE_CURRENT_LIST_DIR}/local_dependency" "-B${TEST_BUILD_DIR}"
+  RESULT_VARIABLE ret
+)
+
+assert_equal(${ret} "0")