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.

include/internal/modules.hpp

@@ -42,6 +42,9 @@ struct dpm_module {
     /** dpm_module_description() as read at load. */
     std::string description;
 
+    /** dpm_module_aliases() as read at load; empty when it returned NULL. */
+    std::string aliases;
+
     /** Resolved dpm_module_execute; the only path into module code. */
     int (*execute)(dpm_ctx*, const char*, int, char**) = nullptr;
 };