Module records and aliases

Installation records what a module reports about itself. dpm_install_module
opens a module once, writes its version and description to a .meta file in
/var/lib/dpm/metadata/, and records the alternate names it declared in
modules.aliases beside it. dpm_uninstall_module removes both, leaving the
module file in place.

dpm_list_modules reads those records and opens no module. A module with no
record lists with a version of <uninstalled> and still loads when a caller
names it. An unreadable or absent metadata directory costs the listing its
detail and costs nothing else.

Aliases give a module alternate names, declared through the new
dpm_module_aliases contract symbol or added with dpm_add_module_alias. A
name is recorded once: one already serving as an alias, or belonging to an
installed module, is refused rather than repointed. dpm_require matches a
name against the installed modules, then the alias table, then the module
path.

The metadata directory is a fifth override field and the -M flag, and
[modules] metadata in core.conf.

The test suite is four binaries covering context, modules, records, and
aliases, each a ctest case of its own, alongside the CLI cases.

tests/fixtures/metadata/good.meta

@@ -0,0 +1,2 @@
+version = 1.2.3
+description = Known-good stub module.

tests/fixtures/metadata/modules.aliases

@@ -0,0 +1,2 @@
+goodie = good
+gd = good

tests/fixtures/src/bad_version.cpp

@@ -28,6 +28,10 @@ extern "C" const char* dpm_module_description(void) {
     return "Fixture with a malformed version.";
 }
 
+extern "C" const char* dpm_module_aliases(void) {
+    return nullptr;
+}
+
 extern "C" int dpm_module_execute(void* ctx, const char* command,
                                   int argc, char** argv) {
     (void)ctx;

tests/fixtures/src/good.cpp

@@ -32,6 +32,12 @@ extern "C" const char* dpm_module_description(void) {
     return "Known-good stub module.";
 }
 
+/* Declares two alternate names, so alias recording is exercised against
+   a module that reports more than one. */
+extern "C" const char* dpm_module_aliases(void) {
+    return "goodie, gd";
+}
+
 /* 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

@@ -2,8 +2,9 @@
  * @file missing_symbols.cpp
  * @brief Broken fixture: exports only part of the module contract
  *
- * Exports the version and description probes and nothing else.
- * libdpm-core.so refuses it at step 1 and names dpm_module_execute.
+ * Exports the version and description probes and nothing else, so two
+ * of the four reserved contract symbols are absent. libdpm-core.so
+ * refuses it at step 1 and names both.
  *
  * Part of the Dark Horse Linux Package Manager (DPM)
  *