One way to record an error, and a name that reads as a query

dpm_module_info_of becomes dpm_get_module_info, joining the other two
readers on the context in stating that a call fetches a value.

Recording an error had two entry points doing the same write: the
exported dpm_set_last_error and an internal dpm_core::set_last_error
taking a std::string. The internal one is gone and the loader calls the
exported function, so a reason reaches the context by one path whether a
module or the library records it.

include/internal/context.hpp

@@ -67,8 +67,8 @@ struct dpm_ctx {
  * @brief Internal implementation of libdpm-core.so
  *
  * Holds what the library's translation units share with each other and
- * with nothing else: the loader, the configuration reader, the error
- * recorder, and the version parser. It is declared in include/internal/,
+ * with nothing else: the loader, the configuration reader, the value
+ * sanitizers, and the version parser. It is declared in include/internal/,
  * which is never installed, so a consumer or a module cannot name any of
  * it.
  *
@@ -82,12 +82,3 @@ struct dpm_ctx {
  * unnamed namespace instead, which gives them internal linkage and keeps
  * them from colliding across files.
  */
-namespace dpm_core {
-    /**
-     * @brief Records a failure reason on the context
-     *
-     * @param ctx The libdpm-core.so context; NULL is a no-op
-     * @param msg The failure description
-     */
-    void set_last_error(dpm_ctx* ctx, const std::string& msg);
-} // namespace dpm_core