cmake: use project-specific root variable instead of CMAKE_SOURCE_DIR

Also applies to *_BINARY_DIR.

This effectively reverts 84083dcc,
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.

Introduce `git_object_rawcontent_is_valid`

Users may want to validate raw object content; provide them a function
to do so.

Return `GIT_EINVALID` on parse errors so that direct callers of parse
functions can determine when there was a failure to parse the object.

The object parser functions will swallow this error code to prevent it
from propagating down the chain to end-users.  (`git_merge` should not
return `GIT_EINVALID` when a commit it tries to look up is not valid,
this would be too vague to be useful.)

The only public function that this affects is
`git_signature_from_buffer`, which is now documented as returning
`GIT_EINVALID` when appropriate.

Minor code cleanups

Fix repo init when template dir is non-existent

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

The filebuf functions should use hashes directly, not indirectly
using the oid functions.

The tests that examine sha1 behavior (including collision detection)
should test against the hash functionality directly, not indirectly
using the oid functions.

Remove some unnecessary includes from utility code.

util: minor cleanup and refactoring to the date class

Instead of `git__date`, just use `git_date`.

cmake: re-enable WinHTTP

Ensure that we `enable_testing()` at the top-level CMakeLists.txt or
else we'll need to navigate within the build directory to the correct
place in the hierarchy to run `ctest`.  Now we can `ctest` at the
top-level again.

tag: set validity to 0 by default

`git_tag_name_is_valid` needs to set validity to 0 when it
short-circuits.

cmake: further refactorings

The select hashes module selects the hash; the CMakeLists.txt selects
the files to implement it.

Update the global variables `LIBGIT2_OBJECTS` to
`LIBGIT2_DEPENDENCY_OBJECTS` for clarity and consistency.

Apply the standard project cmake formatting to the modules.

When building under gcc 11, there is a warning about an incompatible pointer
type, since
[`__atomic_exchange`](https://gcc.gnu.org/onlinedocs/gcc/_005f_005fatomic-Builtins.html)
does not take `volatile` pointers:

```
In file included from ../src/common.h:81,
                 from ../src/transports/winhttp.c:8:
../src/thread-utils.h: In function ‘git___swap’:
../src/thread-utils.h:168:9: warning: argument 3 of ‘__atomic_exchange’ discards ‘volatile’ qualifier [-Wincompatible-pointer-types]
  168 |         __atomic_exchange(ptr, &newval, &foundval, __ATOMIC_SEQ_CST);
      |         ^~~~~~~~~~~~~~~~~
```

This change drops the `volatile` qualifier so that the pointer type matches
what `__atomic_exchange` expects.

When building under gcc 11, there is a warning about a misaligned guard clause
because there were mixed spaces and tabs:

```
[128/634] Building C object src/CMakeFiles/git2internal.dir/threadstate.c.o
../src/threadstate.c: In function ‘threadstate_dispose’:
../src/threadstate.c:39:5: warning: this ‘if’ clause does not guard... [-Wmisleading-indentation]
   39 |     if (threadstate->error_t.message != git_str__initstr)
      |     ^~
../src/threadstate.c:41:9: note: ...this statement, but the latter is misleadingly indented as if it were guarded by the ‘if’
   41 |         threadstate->error_t.message = NULL;
      |         ^~~~~~~~~~~
../src/threadstate.c: At top level:
```

This change indents the code with tabs for consistency with the rest of the
code, which makes the warning go away.

This mimics the behaviour of git which just prints a
warning and continues with the repo initialisation.

Add missing-declarations warning globally

We should enforce declarations throughout the code-base, including
examples, fuzzers and tests, not just in the `src` tree.