PICO_EXTRAS_FETCH_FROM_GIT does not work

[ali1234]

Steps to reproduce:

1. Make a new directory.
2. Put pico_extras_import.cmake in it.
3. Create CMakeLists.txt with the following contents:

include(pico_extras_import.cmake)

4. Create a build directory and enter it.
5. Run cmake -DPICO_EXTRAS_FETCH_FROM_GIT=1 ..

Expected result:

The pico extras repository should be downloaded and added as a subdirectory of the current project.

Actual result:

The pico extras repository is downloaded and then ${CMAKE_BINARY_DIR} is added as a subdirectory. This directory contains no CMakeLists.txt so cmake errors out, and the project cannot be built.

[ali1234]

Observations:

This happens because PICO_EXTRAS_PATH is set to an empty string here and then later made relative to CMAKE_BINARY_DIR here.

The virtually identical code for fetching pico sdk works as expected. The variable PICO_SDK_PATH is set to the actual place where the code was downloaded, even though the code that does it looks identical.

Also note that this isn't because I didn't download the SDK first. Adding the include for it causes it to download 2.2GB of data, and then fail on the extras in the exact same way. I left that step out because downloading 2.2GB is slow and unnecessary to reproduce the problem.