Add support for modular build structure. (#29)

This is part of the effort to make the Boost libraries "modular" for build and consumption. See https://lists.boost.org/Archives/boost/2024/01/255704.php and https://github.com/grafikrobot/boost-b2-modular/blob/b2-modular/README.adoc for more information.

This PR depends on the following other PRs being merged to both develop and master branches of the respective repos:

- https://github.com/boostorg/boost/pull/854

This PR will be changed to ready for review, i.e. not draft, when the above are merged. Do not merge this one until that time.

test/Jamfile.v2

@@ -5,13 +5,15 @@
 # file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
 #
 
+require-b2 5.0.1 ;
+import-search /boost/config/checks ;
+import config : requires ;
 import testing ;
 import feature ;
 
-import ../../config/checks/config : requires ;
-
 project
     : requirements
+        <library>/boost/conversion//boost_conversion
         [ requires cxx11_decltype ]
         # default to all warnings on:
         <warnings>all