[CMAKE] Use CPack to build source packages

Summary:
This diff adds a new package_source build target, which creates a
tar.gz archive of the source files.
The archive contains all the files required to build the software with
autotools or cmake. The output is then slightly different from the
output of autotools' make dist.

Depends on D4267.

Test Plan:

ninja package_source

Use the archive to build the software with autotools and cmake.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D4273