Use Git submodules for custom dependency builds

[khatchad]

Currently, there are two custom dependency builds:

https://github.com/wala/ML/blob/40fae525a98e73a1424132f852f3ca39784a6a7d/.github/workflows/continuous-integration.yml#L19-L40

Currently, these dependencies are cloned from GH from master. It would be better instead to specify a specific SHA1 to be checked out as to stabilize the build. Moreover, to build from source, users must examine the build file or https://github.com/wala/ML/blob/master/CONTRIBUTING.md.

I believe that both of these issues may be helped using Git submodules. There, you can specify the clone URL and SHA1 to be used, and the submodules can be cloned along with the main repo. There is also a GH action for submodules (see https://github.com/actions/checkout#user-content-usage).