Make output validate paths are absolute and normalized

Also update the checks in Symlink to use the same functions

omicron/ssg/output/output.py

@@ -9,6 +9,10 @@ else:
     Site = "omicron.ssg.site.Site"  # for runtime checking with beartype
 
 
+def is_normalized(path: Path) -> bool:
+    return ".." not in path.parts
+
+
 class OutputError(RuntimeError):
     pass
 
@@ -16,7 +20,12 @@ class OutputError(RuntimeError):
 class Output(ABC):
     def __init__(self, site: Site, destination: Path):
         self._site_ref = weakref.ref(site)
+        if destination.is_absolute():
+            raise OutputError("destination path must be relative")
+        if not is_normalized(destination):
+            raise OutputError("destination path must be normalized")
         self.destination = destination
+
         url = site.base_dir + "/" + destination.as_posix()
         url = url.removesuffix("/index.html") or "/"
         self.url = url

omicron/ssg/output/symlink.py

@@ -1,7 +1,7 @@
 import logging
 import os
 from typing import TYPE_CHECKING
-from omicron.ssg.output.output import Output, OutputError
+from omicron.ssg.output.output import Output, OutputError, is_normalized
 from pathlib import Path
 
 log = logging.getLogger(__name__)
@@ -15,13 +15,9 @@ else:
 class Symlink(Output):
     def __init__(self, site: Site, link: Path, target: Path):
         if target.is_absolute():
-            raise ValueError("Symlinks must be relative")
-        if (
-            not (site.output_path / target)
-            .resolve()
-            .is_relative_to(site.output_path.resolve())
-        ):
-            raise ValueError("Symlink target escapes the output directory")
+            raise OutputError("Symlinks target must be relative")
+        if not is_normalized(target):
+            raise OutputError("Symlinks target must be normalized")
         super().__init__(site, link)
         self.target = target