Improved path handling

This commit fixes a bunch of issues at the same time, as they are all
related to path handling:

- `pathlib.Path.resolve()` or `.absolute()` are replaced by
  `bpathlib.make_absolute()`. The latter does NOT follow symlinks and does
  NOT network mounts from a drive letter to UNC notation. This also has
  advantages on non-Windows sytems, as it allows BAT-packing a directory
  structure with symlinked files (such as a Shaman checkout).
- Better handling of drive letters, and of paths that cross drive
  boundaries.
- Better testing of Windows-specific cases when running the tests on
  Windows, and of POSIX-specific cases on other platforms.

Thanks to @wisaac for starting this patch in D6676.

Thanks to @jbakker for pointing out the drive letter issue. This fixes
T70655.

blender_asset_tracer/blendfile/__init__.py

@@ -49,7 +49,7 @@ def open_cached(path: pathlib.Path, mode='rb',
                 assert_cached: typing.Optional[bool] = None) -> 'BlendFile':
     """Open a blend file, ensuring it is only opened once."""
     my_log = log.getChild('open_cached')
-    bfile_path = path.absolute().resolve()
+    bfile_path = bpathlib.make_absolute(path)
 
     if assert_cached is not None:
         is_cached = bfile_path in _cached_bfiles
@@ -84,13 +84,13 @@ def close_all_cached() -> None:
 
 def _cache(path: pathlib.Path, bfile: 'BlendFile'):
     """Add a BlendFile to the cache."""
-    bfile_path = path.absolute().resolve()
+    bfile_path = bpathlib.make_absolute(path)
     _cached_bfiles[bfile_path] = bfile
 
 
 def _uncache(path: pathlib.Path):
     """Remove a BlendFile object from the cache."""
-    bfile_path = path.absolute().resolve()
+    bfile_path = bpathlib.make_absolute(path)
     _cached_bfiles.pop(bfile_path, None)