Assert that the repo directory is named `executorch` (#6480)

Our C++ include path assumes that the repo lives in a directory named exactly `executorch`. Users who name it something else run into hard-to-debug issues (see #6475).

Since we require it, add an explicit check and point users to the issue that tracks the fix.

## Test Plan

Tried generating cmake files under a directory named `executorch` and named `not-executorch`:

```
(rm -rf cmake-out \
    && mkdir cmake-out \
    && cd cmake-out \
    && cmake ../)
```

Note that I added a trailing slash just in case CMAKE_CURRENT_SOURCE_DIR included that slash and might confuse `cmake_path()`. (note that CMAKE_CURRENT_SOURCE_DIR does not include the trailing slash in this case)

This command succeeded when the directory is named `executorch`.

Under `not-executorch` it failed with the error:

```
CMake Error at CMakeLists.txt:332 (message):
  The ExecuTorch repo must be cloned into a directory named exactly
  `executorch`; found `not-executorch`.  See
  https://github.com/pytorch/executorch/issues/6475 for progress on a fix for
  this restriction.
```

Also tested with the instructions at [pytorch.org/executorch/main/llm/getting-started.html](https://pytorch.org/executorch/main/llm/getting-started.html), where executorch is a sub-repo of the top project. It builds when following the directions, but if I rename the repo to third-party/not-executorch then I see the expected error.