build, refactor: Improve package version usage

Summary:

build, refactor: Use conventional version notation for boost package

Dot is used as a separator in versions of other depends packages.

build, refactor: Reuse expat package version in its download path

This is a backport of core#24276

Depends on D13983

Test Plan: guix and gitian builds

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13984