Merge bitcoin/bitcoin#24276: build, refactor: Improve package version usage

edc9a6a build, refactor: Reuse expat package version in its download path (Hennadii Stepanov)
4bb7821 build, refactor: Use conventional version notation for boost package (Hennadii Stepanov)

Pull request description:

  `boost` package:
  - `.` is used as a separator in versions of other depends packages.

  `expat` package:
  - reuse package version in its download path

  ---

  The straightforward way to verify this PR:
  ```
  $ cd depends
  $ make clean-all
  $ make boost_fetched
  $ make expat_fetched
  ```

ACKs for top commit:
  prusnak:
    ACK edc9a6a
  shaavan:
    ACK edc9a6a

Tree-SHA512: c15d672fe34ac59850425d3d6a6eee5f720e16d227aad1332a563b218465879b7ee6fb865dd1bac06aedf356f9bb1c67112d9d88da8f877f04838b50a9dc97be

Author: fanquake

depends/packages/boost.mk

@@ -1,7 +1,7 @@
 package=boost
-$(package)_version=1_71_0
-$(package)_download_path=https://boostorg.jfrog.io/artifactory/main/release/$(subst _,.,$($(package)_version))/source/
-$(package)_file_name=boost_$($(package)_version).tar.bz2
+$(package)_version=1.71.0
+$(package)_download_path=https://boostorg.jfrog.io/artifactory/main/release/$($(package)_version)/source/
+$(package)_file_name=boost_$(subst .,_,$($(package)_version)).tar.bz2
 $(package)_sha256_hash=d73a8da01e8bf8c7eda40b4c84915071a8c8a0df4a6734537ddde4a8580524ee
 $(package)_dependencies=native_b2
 $(package)_patches=fix_openbsd_test_lib.patch

depends/packages/expat.mk

@@ -1,6 +1,6 @@
 package=expat
 $(package)_version=2.4.1
-$(package)_download_path=https://github.com/libexpat/libexpat/releases/download/R_2_4_1/
+$(package)_download_path=https://github.com/libexpat/libexpat/releases/download/R_$(subst .,_,$($(package)_version))/
 $(package)_file_name=$(package)-$($(package)_version).tar.xz
 $(package)_sha256_hash=cf032d0dba9b928636548e32b327a2d66b1aab63c4f4a13dd132c2d1d2f2fb6a