Improve Submodule

[heavenfall]

Current submodule not simple. Issues listed below:

• Submodule system requires extra commands
• Submodules are not modifiable by default
• Limited instruction setup (requires knowledge on git and cmake)
• No example of basic cmake file or setup commands explicitly listed
• Bug in setting up own git repo with subtree (e.g. looks for extern/warthog-core/.git found in submodules but not subtree)

[heavenfall]

Branch enhancement/submodule has improved readme for submodule and subtree, with example project setup and CMake file.

Rename cmake/submodule.cmake to cmake/warthog.cmake.

Submodule currently uses warthog_submodule, which now correctly looks for CMakeFiles.txt in the expected extern directory for submodule.

[heavenfall]

To address the issue of easy setup, a fallback on FetchContent is a good methodology.

Pros:
It easily handles diamond inheritance.
It is simple.
The user could just fetch the required modules in their CMakeFiles.txt and not worry about git submodule/subtree.

Cons:
It clones into CMAKE_BINARY_DIR thus is not modifiable by the user.
It is hidden, only seen when cmake is run.

Propisition
We can make FetchContent a fallback.
Have warthog_submodule function find module as follows:

1. if WARTHOG_MODULE_name set? already included, this will be set at the module level
2. if ${CMAKE_SOURCE_DIR}/extern/CMakeFiles.txt exists?
   i. is submodule or subtree at this level, include
   ii. question on if this should only be allowed on top-level project or in the submodule level is debatable, right now is governed by CMake variable the user can set.
3. FetchContent
   i. is fallback
   ii. user can override default FetchContent as is first come basis
   iii. for (2), top-level makes sense and we now where to check it packaged, so avoid issue of diamond pattern where it first checks one and fetches, then finds the 2nd that depends on Warthog-Core that is available at that level but too late.

The FetchContent should be a backup, so the user should be able to do a git submodule init, and it will not be invoked.

[heavenfall]

Submodule system reworked and in release #46