std.Build.Dependency: add optional~ functions

[dasimmet]

When a std.Build.Dependency relies on a transitive b.lazyDependency to expose an artifact, module, namedWriteFiles or namedLazyPath, calls to these functions panic when the dependency was not downloaded yet, but work fine when it is available in the local cache.

We can prevent this by using optionalArtifact, returning an optional instead of panic.
Same for optionalModule , optionalNamedWriteFiles and optionalNamedLazyPath.
I think the optional~ prefix is better than lazy~ here to avoid confusion with LazyPath.

The downside I guess is it could lead to dead code if the named Item will never be available...