- projectile-get-project-directories: replace the (or keep '("")) trick
  with an explicit (if keep ... (list project-dir)). The behavior is
  unchanged but the empty-keep case no longer reads as a string-concat
  with the empty string.

- projectile-dirconfig-file: expand the docstring to spell out the
  dual marker/config role — empty file is enough to mark a project,
  non-empty content drives parsing.

The cache value used to be (MTIME . PARSED-RESULT) keyed on the
project root. If the user changed `projectile-dirconfig-file' to
point at a different file mid-session, the old entry kept getting
returned because the key didn't depend on the path. Store the path
alongside the mtime in the cached value and compare both before
considering a hit. The hash key remains the project root so
`projectile-invalidate-cache' continues to work as-is.

The other two dirconfig warnings (alien-mode bypass and prefix-less
entries) are gated by per-project hash sets so they fire at most
once per Emacs session. The glob-keep warning was the odd one out —
it lived inside the uncached parser and re-fired on every cache
miss, and it emitted one display-warning per offending entry rather
than a single consolidated message.

Move it into the cached wrapper alongside the others, gate it on
projectile--glob-keep-warned-projects, and roll multiple offending
entries into one warning that lists them.

Refactor dirconfig parser: struct, pure classifier, deprecation warning

The implicit "any unprefixed line is an ignore pattern" rule is the
last source of subtle parser surprises — it's the reason why a
single leading space silently changes a +-keep into a literal ignore
pattern, and it makes typo'd comments slip through as ignores.

Mark these lines as :legacy-ignore in the classifier, record them
in a new prefixless-ignore slot on the dirconfig struct, and emit a
one-time warning per project listing the offending entries. The
behavior is unchanged — the lines still go into the ignore list —
but users now get a nudge to write them as -entry. The warning can
be silenced via projectile-warn-on-prefixless-dirconfig-lines.

The old parser walked a temp buffer with point and pcase'd on
char-after, mixing IO, prefix dispatch, and bucket bookkeeping into
one function. Pull the dispatch out into projectile--dirconfig-classify-line,
which takes a string and returns a (BUCKET . VALUE) tag. The pure
function is unit-testable without buffer plumbing, the IO wrapper
shrinks to a one-shot read + dispatch, and the awkward
(pcase ((pred (lambda ...)) ...)) for the comment-prefix check
becomes a straightforward cond.

No behavior change.

Replace the positional (KEEP IGNORE ENSURE) triple with a
cl-defstruct. Every internal call site used car/cadr/caddr to pull
out a slot, which is unreadable and error-prone — slot accessors
make the intent explicit and let cl-defstruct grow a fourth field
later without touching every consumer.

Existing callers that compared against the raw triple (a couple of
internal helper tests) are updated to construct the struct with
make-projectile-dirconfig.

Improvements to .projectile (dirconfig) parsing and ergonomics

The parser silently turns every keep entry into a directory via
file-name-as-directory, which means a user-typed +*.json or +/foo.txt
becomes "*.json/" or "foo.txt/" and quietly never matches anything.
Spot the obvious misuses (anything containing *, ?, or [) at parse
time and emit a warning so the user can correct the file or move the
pattern to an ignore/ensure rule.

The existing parser tests stub insert-file-contents; nothing exercises
the full IO path. Add three sandbox-based tests: a mixed +/-/!/no-prefix
file, a non-ASCII round-trip, and a file with no trailing newline. These
guard against IO-layer regressions that the stubbed tests can't see.