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.

CMakeLists.txt

@@ -21,9 +21,11 @@ set(CMAKE_CXX_STANDARD_REQUIRED ON)
 # libdpm-core.so — the library (C ABI)
 # ---------------------------------------------------------------------
 add_library(dpm-core SHARED
+        src/core/aliases.cpp
         src/core/conf.cpp
         src/core/context.cpp
         src/core/logging.cpp
+        src/core/metadata.cpp
         src/core/modules.cpp
         src/core/sanitizers.cpp
         src/core/version.cpp