Also applies to *_BINARY_DIR.

This effectively reverts 84083dcc8bd41332ccac9d7b537f3e254d79011c,
which broke all users of libgit2 that use it as a CMake subdirectory
(via `add_subdirectory()`). This is because CMAKE_SOURCE_DIR refers
to the root-most CMake directory, which in the case of
`add_subdirectory()` is a parent project to libgit2 and thus the paths
don't make any sense to the configuration files. Corollary,
CMAKE_SOURCE_DIR only makes sense if the CMake project is always the
root project - which can rarely be guaranteed.

In all honesty, CMake should deprecate and eventually remove
CMAKE_SOURCE_DIR and CMAKE_BINARY_DIR. It's been the source of headaches
and confusion for years, they're rarely useful over
CMAKE_CURRENT_(SOURCE|BINARY)_DIR or PROJECT_(SOURCE|BINARY)_DIR,
and they cause a lot of confusing configuration and source
code layouts to boot.

Any time they are used, they break `add_subdirectory()` almost 100% of
the time, cause confusing error messages, and hide subtle bugs.

Instead of using the project-specific `libgit2_SOURCE_DIR` and
`libgit2_BINARY_DIR` variables, use `CMAKE_SOURCE_DIR` and
`CMAKE_BINARY_DIR`.

Ensure that we always use lowercase function names, and that we do not
have spaces preceding open parentheses, for consistency.

While our contribution guide tries to make clear the licenses that apply
to libgit2, it does not make clear that different licenses apply to our
bundled dependencies. Make this clear by listing each dependency
together with the licenses that they are governed by. Furthermore,
bundle the complete license texts next to the code they apply to.

Include the constants for `WINHTTP_FLAG_SECURE_PROTOCOL_TLS1_1` and
`WINHTTP_FLAG_SECURE_PROTOCOL_TLS1_2` so that they can be used by mingw.

This updates both the `deps/winhttp` framework (for classic mingw) and
adds the defines for mingw64, which does not use that framework.

Due to our split of CMake files into multiple modules, we had to replace
some uses of the `${CMAKE_CURRENT_SOURCE_DIR}` and
`${CMAKE_CURRENT_BINARY_DIR}` variables and replace them with
`${CMAKE_SOURCE_DIR}` and `${CMAKE_BINARY_DIR}`. This enabled us to
still be able to refer to top-level files when defining build
instructions inside of a subdirectory.

When replacing all variables, it was assumed that the absolute set of
variables is always relative to the current project. But in fact, this
is not the case, as these variables always point to the source and
binary directory as given by the top-levl project. So the change
actually broke the ability to include libgit2 directly as a subproject,
as source files cannot be found anymore.

Fix this by instead using project-specific source and binary directories
with `${libgit2_SOURCE_DIR}` and `${libgit2_BINARY_DIR}`.

Extract code required to build the winhttp library into its own
CMakeLists.txt, which is included as required.