Let a caller ask which targets a configuration declares

[ps-pufferfish]

Why

A caller that generates one configuration into several places has somewhere to put each target, and so needs to know which targets there are. relcoord is that caller: it holds a manifests directory per cluster and generates a target into each.

With no way to ask, it opens config.toml itself and composes find_config_file, load_toml_file, config_version and parse_targets into a small loader of its own — this package's business, in another repository. Two costs: it has to keep its own idea of what a target is, and it can reach a different conclusion about a configuration than generate() does. It also has to import parse_targets, which reads more like an internal of the loader than a contract.

declared_targets(config_dir) is the question those four calls were spelling out:

from manifest_builder import declared_targets

declared_targets(Path("conf"))   # ("platform-dev", "platform-prod")

Behaviour

• The declared names, in the order the file declares them.
• None for a directory that declares config blocks directly, and for one holding no config file at all — which generate() reports on better than a version check would.
• A malformed target raises here the same way generating it would, since it is parse_targets that decides.

That None also answers the question the caller was really asking. generate() takes target or vars and rejects the wrong one for the layout, so a caller has to know which layout the file is before it can call at all — which is why relcoord was reading the file in the first place. One call now covers both.

What it lets the caller stop doing

portswigger-cloud/relcoord#98 needs this. That PR is about a related problem — relcoord asks each configured output for the target of its own name, so a config repository declaring only the clusters it deploys to fails with Unknown target on the rest — and on top of this it shrinks to one call in place of a four-call composition and a version branch.

Verification

276 passed; ruff check, ruff format --check and ty check clean. Six new tests: the names in declared order, None for both block-style layouts and for a missing file, a manifest-builder.toml, and a target without sections failing the way generating it would.

README gains a short Asking which targets a configuration declares subsection under Targets and sections.