build.jam: fix malformed alias so b2 install generates cmake config

The alias used `usage-requirements` as a literal source name instead of
the dedicated 5th argument position, and lived outside the `explicit`
block. As a result `boost-library openmethod` could not detect the
library's interface and `b2 install` skipped the cmake config files.
Downstream consumers calling `find_package(Boost CONFIG REQUIRED
COMPONENTS openmethod)` against a b2-installed Boost (e.g. the
unified-cmake CI job) then failed with:

  Could not find a package configuration file provided by
  "boost_openmethod" (requested version 1.92.0)

Move the alias inside `explicit` and put `<include>include
<library>$(boost_dependencies)` in the usage-requirements slot,
matching the pattern used by every other header-only Boost library
(assert, core, describe, mp11, preprocessor, ...).

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

Author: jll63

build.jam

@@ -15,12 +15,9 @@ constant boost_dependencies
 
 project /boost/openmethod ;
 
-alias boost_openmethod
-    : usage-requirements
-      <include>include
-      <library>$(boost_dependencies)
+explicit
+    [ alias boost_openmethod : : : : <include>include <library>$(boost_dependencies) ]
+    [ alias all : boost_openmethod test ]
     ;
-alias all : boost_openmethod test ;
-explicit all ;
 
 call-if : boost-library openmethod ;