Modules determine their own compatibility with the library

A module is built against the system-installed libdpm-core.so and is
responsible for being correct against it. Where it needs to act on the
version it is running under, dpm_core_version() reports that and the
module decides for itself.

dpm_module_core_min() is removed. It was a declaration handed to the
library to enforce on the module's behalf, and enforcement of that kind
belongs nowhere in a library that routes and hosts. The contract is now
three reserved symbols and load validation is two steps: the reserved
symbols resolve, and the version and description probes return
well-formed values.

compare_versions had no remaining caller and is removed; parse_version
stays for the well-formedness probe. The core_too_new fixture went with
the handshake it existed to exercise.

tests/fixtures/src/bad_version.cpp

@@ -30,11 +30,6 @@ extern "C" const char* dpm_module_description(void)
     return "Fixture with a malformed version.";
 }
 
-extern "C" const char* dpm_module_core_min(void)
-{
-    return "0.1.0";
-}
-
 extern "C" int dpm_module_execute(void* ctx, const char* command,
                                   int argc, char** argv)
 {

tests/fixtures/src/core_too_new.cpp

@@ -1,47 +0,0 @@
-/**
- * @file core_too_new.cpp
- * @brief Broken fixture: demands a libdpm-core newer than any that exists
- *
- * Contract-complete, but dpm_module_core_min() reports 99.0.0.
- * libdpm-core must refuse it at validation step 2 with a message
- * naming the remedy (update libdpm-core).
- *
- * Part of the Dark Horse Linux Package Manager (DPM)
- *
- * This program is free software: you can redistribute it and/or modify
- * it under the terms of the GNU Affero General Public License as
- * published by the Free Software Foundation, either version 3 of the
- * License, or (at your option) any later version.
- *
- * This program is distributed in the hope that it will be useful,
- * but WITHOUT ANY WARRANTY; without even the implied warranty of
- * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
- * GNU Affero General Public License for more details.
- *
- * You should have received a copy of the GNU Affero General Public License
- * along with this program. If not, see <https://www.gnu.org/licenses/>.
- */
-extern "C" const char* dpm_module_version(void)
-{
-    return "1.0.0";
-}
-
-extern "C" const char* dpm_module_description(void)
-{
-    return "Fixture demanding a future libdpm-core.";
-}
-
-extern "C" const char* dpm_module_core_min(void)
-{
-    return "99.0.0";
-}
-
-extern "C" int dpm_module_execute(void* ctx, const char* command,
-                                  int argc, char** argv)
-{
-    (void)ctx;
-    (void)command;
-    (void)argc;
-    (void)argv;
-    return 0;
-}

tests/fixtures/src/good.cpp

@@ -34,11 +34,6 @@ extern "C" const char* dpm_module_description(void)
     return "Known-good stub module.";
 }
 
-extern "C" const char* dpm_module_core_min(void)
-{
-    return "0.1.0";
-}
-
 /* Answers "ping" with 42 so a dispatch round trip is observable, and
    0 for anything else. */
 extern "C" int dpm_module_execute(void* ctx, const char* command,

tests/fixtures/src/missing_symbols.cpp

@@ -2,9 +2,8 @@
  * @file missing_symbols.cpp
  * @brief Broken fixture: exports only part of the module contract
  *
- * Missing dpm_module_execute, dpm_module_core_min, and
- * dpm_module_manifest. libdpm-core must refuse it at step 1 and
- * name the missing symbols.
+ * Exports the version and description probes and nothing else.
+ * libdpm-core.so refuses it at step 1 and names dpm_module_execute.
  *
  * Part of the Dark Horse Linux Package Manager (DPM)
  *