populate: stay inside the target directory

Populate replaces what's at the target paths of sources, removes what
sources don't deliver anymore, and prunes.  Nothing of that may reach
beyond the target directory, whatever names sources have and whatever the
target directory contains:

- Source names that aren't file names in the target directory don't
  evaluate: the empty name, ., .., .populate, and names with slashes or
  control characters.  E.g. a git source named "" removes the target
  directory, and a file source named .. syncs into its parent.
- Target paths that aren't absolute, that are the root directory, or that
  contain .. don't evaluate.
- A symlink at the target path of a source gets replaced, not followed,
  and populate says so.  rsync-based and flake sources do that already.
  git sources replace the link as well, and also a work tree whose .git is
  a symlink or a file naming a repository elsewhere, instead of fetching,
  resetting, and cleaning there.  derivation sources replace a link that
  doesn't point into the store, instead of failing.
- Populate refuses to use the sentinel, or a state directory therein, if
  it is a symlink, instead of changing and removing what it points to.
- What populate removes recursively gets removed as a link if it is one:
  chmod -R follows a link named on the command line.
- Flake and flake-input sources that get patched, get their lock file
  rewritten, or are materialized replace a symlink at their target path
  like every source does, and say so.
- Nix rewrites lock files in place, which changes other hard links to
  them as well, e.g. in a backup or outside the target directory, and
  what a lock file that is a symlink points to.  Populate replaces a lock
  file by a copy of its own before rewriting it.

The target directory itself may be a symlink.  Nix refuses path: URLs with
symlinks on the way, so the flakes step resolves the paths it hands to
Nix.

Backups work on a sentinel that is still a file, which becomes a
directory, like for everything else that keeps state.

The tests create canaries outside the target directories: a file and a
directory beside them, and a directory elsewhere with a read-only
directory and a Git work tree with uncommitted changes.  The check of
every phase of every scenario fails if a canary changed, or if a store
path that the target directory links to doesn't match its hash anymore.
A new test, safety, points links at the canaries: at the target path of
every source type, inside what sources replace, in place of populate's
state, and among what it removes and prunes.  It also covers the names
and target paths that don't evaluate, a target directory that is a
symlink, and a lock file with another hard link.

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