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.

docs/PROSE/OVERVIEW.md

@@ -51,7 +51,7 @@ The target root override is what makes chroot builds, image assembly, and sysroo
 
 The context owns everything it hands out. Every string a caller receives stays valid until `dpm_close`, and callers never free anything.
 
-**Acquiring a module.** `dpm_require` resolves a module by name, validates it completely, and returns a handle — or returns NULL, with `dpm_last_error` carrying the precise reason. Modules load at most once per context, and repeated calls return the same handle.
+**Acquiring a module.** `dpm_require` resolves a module by name, validates it completely, and returns a handle — or returns NULL, with `dpm_get_last_error` carrying the precise reason. Modules load at most once per context, and repeated calls return the same handle.
 
 **Reading what the library saw.** `dpm_module_info_of` fills in a module's name, version, and description. Those values are reported, and no conclusion is drawn from them.
 
@@ -59,7 +59,7 @@ The context owns everything it hands out. Every string a caller receives stays v
 
 **Enumerating.** `dpm_list_modules` yields a cursor over every valid module, which is what backs the listing the `dpm` binary prints.
 
-**Services.** A module reaches the library through the context that dispatched the call: `dpm_log` to write a message, `dpm_config_get` to read a value from its own configuration namespace, `dpm_module_path` to learn where modules live, `dpm_core_version` to learn the library's version. A module needs no file handling and no logging machinery of its own.
+**Services.** A module reaches the library through the context that dispatched the call: `dpm_log` to write a message, `dpm_config_get` to read a value from its own configuration namespace, `dpm_get_resolved_module_path` to learn where modules live, `dpm_core_version` to learn the library's version, `dpm_set_last_error` to record why it failed. A module needs no file handling and no logging machinery of its own.
 
 ## How a Module Reaches Another Module
 
@@ -70,7 +70,7 @@ int dpm_module_execute(dpm_ctx* ctx, const char* command, int argc, char** argv)
 {
     dpm_module* peer = dpm_require(ctx, "othermodule");
     if (!peer) {
-        dpm_log(ctx, DPM_LOG_ERROR, dpm_last_error(ctx));
+        dpm_log(ctx, DPM_LOG_ERROR, dpm_get_last_error(ctx));
         return 1;
     }
     return dpm_execute(ctx, peer, "somecommand", argc, argv);