Include files under symlinked directories in the sdist

[faresrafat3]

Fixes #509.

A directory symlink nested inside an included directory was silently dropped
from the sdist, so this pyproject.toml:

[tool.flit.sdist]
include = ["docs/"]

with this tree:

docs/
  linked -> ../real_data      # symlink
real_data/
  data.txt

produced an sdist without docs/linked/data.txt. Reproduced by building the
sdist and listing the tar members — the file is absent.

Root cause

SdistBuilder.apply_includes_excludes collects the contents of each included
directory with os.walk, which does not follow directory symlinks by
default:

for dirpath, dirs, dfiles in os.walk(osp.join(cfgdir_s, rel_d)):

Note that a top-level include = ["linked_data/"] naming the symlink directly
does work (the pattern matches the symlink itself), which is why the bug only
shows up for a symlink nested inside a walked directory.

Change

Walk with followlinks=True. On its own that would recurse forever on a
symlink cycle, so the real path of every visited directory is recorded and
repeats are pruned:

seen_dirs: set[str] = set()
for dirpath, dirs, dfiles in os.walk(
    osp.join(cfgdir_s, rel_d), followlinks=True
):
    real = os.path.realpath(dirpath)
    if real in seen_dirs:
        dirs[:] = []
        continue
    seen_dirs.add(real)

Verification

Built the sdist before and after:

tree	before	after
docs/linked -> real_data	file missing	docs/linked/data.txt present
docs/sub/loop -> docs (cycle)	n/a	terminates, 4 members, no duplicates

The cycle case was measured explicitly because followlinks=True is unsafe
without the guard — with the naive one-line change it produced 45 members with
deeply repeated loop/sub/loop/sub/... paths before the seen-set was added.

Two regression tests added to tests/test_sdist.py, both driving
apply_includes_excludes directly with a fixed file list so they do not depend
on git/hg being present: one asserts the symlinked directory's file is
included, the other asserts a symlink cycle terminates with no duplicate
entries.

Full suite: 353 passed, 1 failed — the failure (tests/test_upload.py::test_upload)
is pre-existing and reproduces identically on the unmodified tree (it needs the
flit CLI installed, which this environment lacks).

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[faresrafat3]

Hi — following up on this PR. It includes files under symlinked directories in the sdist. Let me know if you'd like any changes or if there's anything I can help with.