Issue Description:

After playing around with binary frameworks, I tried to use the `compute-checksum` command on an `xcframework` directory. The resulting error printed was:

"file not found at path: <my_path>"

I was a bit confused and double-checked the path exists before realizing it was expecting a file and not the directory.

Since it may be common to pass in an `xcframework` here, I propose adding a small diagnostic giving a hint that a `zip` file is expected.

I have a small commit that implements this at db17270 and I'm happy to open a PR if that's desired and a proper way forward.

Thanks!