libponyc: resolve relative paths in use "path:..." statements

The use of `pony_realpath` (which calls the POSIX `realpath` underneath)
is convenient, but brings a change in behaviour: before, we have passed
just about anything into `prog->libpaths`, regardless of the directory's
existence. With `realpath`, nonexistent directories will make the call
return an error.

To remediate the impact a bit, I've added an `errorf()` statement.

However, to make this round, I think we might have to either check for
existence in both cases, or don't do that in either one.

Signed-off-by: Stephan Renatus <srenatus@chef.io>

src/libponyc/pkg/program.c

@@ -148,14 +148,40 @@ bool use_path(ast_t* use, const char* locator, ast_t* name,
   pass_opt_t* options)
 {
   (void)name;
+  char resolved[FILENAME_MAX];
+  char buf[FILENAME_MAX];
+
+  if(is_path_absolute(locator)) {
+    if(strlen(locator) > FILENAME_MAX)
+      return false;
+
+    strcpy(resolved, locator);
+  } else {
+    // resolve path relative to pkg path
+    ast_t* pkg_ast = ast_nearest(use, TK_PACKAGE);
+    const char* pkg_path = package_path(pkg_ast);
+
+    if(strlen(pkg_path) + strlen(locator) >= FILENAME_MAX)
+      return false;
+
+    strcpy(buf, pkg_path);
+    strcat(buf, "/");
+    strcat(buf, locator);
+
+    if(pony_realpath(buf, resolved) != resolved) {
+      errorf(options->check.errors, package_filename(pkg_ast),
+             "resolve \"path:%s\": %s", locator, strerror(errno));
+      return false;
+    }
+  }
 
-  const char* libpath = quoted_locator(options, use, locator);
+  const char* libpath = quoted_locator(options, use, resolved);
 
   if(libpath == NULL)
     return false;
 
-  ast_t* p = ast_nearest(use, TK_PROGRAM);
-  program_t* prog = (program_t*)ast_data(p);
+  ast_t* prog_ast = ast_nearest(use, TK_PROGRAM);
+  program_t* prog = (program_t*)ast_data(prog_ast);
   pony_assert(prog->lib_args == NULL); // Not yet built args
 
   if(strlist_find(prog->libpaths, libpath) != NULL) // Ignore duplicate