Modules can report why they failed

A module had one channel back to its caller: the int returned from
dpm_module_execute, handed through by dpm_execute. Any detail behind
that number could only reach a log, so a caller wanting the reason had
to read output rather than ask for it.

dpm_set_last_error joins the exported API. A module records its reason
on the context handed to its entry point, which belongs to the caller,
and the caller reads it back with dpm_get_last_error.

The two accessors on the context now say what they do to it:
dpm_module_path becomes dpm_get_resolved_module_path, naming the value
it reports rather than the setting it came from, and dpm_last_error
becomes dpm_get_last_error, pairing with the setter.

Path normalization moves to sanitizers.cpp, which holds the conversions
that put a value written by a person into the single form the library
stores it in.

tests/test_core.cpp

@@ -51,7 +51,7 @@ static int g_checks   = 0;
     } while (0)
 
 static bool error_contains(dpm_ctx* ctx, const char* needle) {
-    const char* err = dpm_last_error(ctx);
+    const char* err = dpm_get_last_error(ctx);
     return err != nullptr && std::strstr(err, needle) != nullptr;
 }
 
@@ -88,7 +88,7 @@ int main(void) {
         CHECK(dpm_config_get(ctx, "nomodule", "main", "key") == nullptr);
 
         /* module-path override wins over the config value */
-        const char* path = dpm_module_path(ctx);
+        const char* path = dpm_get_resolved_module_path(ctx);
         CHECK(path != nullptr &&
               std::strncmp(path, TEST_FIXTURE_MODULES,
                            std::strlen(TEST_FIXTURE_MODULES)) == 0);