do_adjust_git: FileExistsError on dangling symlinks when DL_DIR path changes

[timblaktu]

Summary

do_adjust_git() in meta/classes-recipe/dpkg-base.bbclass fails with
FileExistsError when DL_DIR changes between kas-container sessions,
leaving a dangling symlink at ${WORKDIR}/git.

Root Cause

Lines 30 and 33 use os.path.exists(), which follows symlinks. When the
symlink target no longer exists (dangling), os.path.exists() returns
False — even though the symlink itself still exists on disk.

This causes:

1. Line 30: The stale symlink check (if os.path.exists(git_link) and ...)
   evaluates to False, so the dangling symlink is not removed.
2. Line 33: The creation check (if not os.path.exists(git_link)) evaluates
   to True, so os.symlink() attempts to create a new symlink — but
   the old one still exists, raising FileExistsError.

Reproduction

1. Run an ISAR build with DL_DIR=/path/a
2. Change DL_DIR to /path/b (e.g., kas-container mounts change between sessions)
3. Run the build again — do_adjust_git fails with:

   FileExistsError: [Errno 17] File exists: '/path/b/git' -> '${WORKDIR}/git'
   

Fix

Replace os.path.exists() with os.path.lexists() on both lines.
os.path.lexists() checks for the existence of the path itself (the symlink
entry) rather than following the symlink to its target.