interpreter: load data files with an explicit ?format= override

[daandemeyer]

Loading static data with load() required the file to be named .json or
.toml, because the extension was the only thing selecting a parser. That rules
out the files people actually want to read: Cargo.lock is TOML, a
package.json.tmpl is JSON, and neither can be renamed.

Add a ?format= query on the load path (load(":deps.lock?format=toml", ...))
accepting bzl, json, or toml. It cannot be a load() kwarg: format = "toml" is already valid syntax meaning "alias the local name format to the
exported symbol toml", so taking that spelling would need a grammar change and
would silently change the meaning of existing code. The ? was already reserved
by ImportPath.

The format is stored on ImportPath and participates in its hash and equality,
so it is part of the DICE key rather than a side channel. That makes the loaded
file a tracked dependency of the parse and gets invalidation for free, and the
extension check is skipped exactly when a format was given. Every site that
dispatched on the extension now asks ImportPath::load_format() instead.

A format that merely restates the extension collapses to None, so
:a.bzl?format=bzl and :a.bzl stay one module. Otherwise the same file would
be evaluated under two keys and hand out two copies of its providers and
transitive sets, breaking the pointer-identity checks that resolve_load
already warns about.

An unparseable file stays a load-time error, which fails the whole package.
That is inherent: a parse that is not at load time is not a tracked dependency.
Since the extension no longer implies the parser, the error now names the format
and where it came from. The blast radius is documented, with a pointer to
reading the file in an action when failure needs to be scoped to its consumers.

Incidentally fixes prelude .toml files being evaluated as Starlark: the
prelude branch special-cased only .json, and now shares the format dispatch.

Covered by unit tests for format inference, the relaxed extension check, the
redundant-format collapse and display round-tripping, plus e2e tests for the
three formats, invalidation on edit, and each error path.

Signed-off-by: Daan De Meyer daan@amutable.com

[meta-codesync]

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