We should hold off actually building the binary module file until it is imported.
`canImport` queries can happen, for example, during dependency scanning, when we do not wish to have the scanner tool execute any module builds.
Resolves rdar://82603098
This matches the behavior of the current client (`swift-driver`) and reduces ambiguity in how the nodes in the graph are to be treated. Swift dependencies with a textual interface, for example, must be built into a binary module by clients. Swift dependencies without a textual interface, with only a binary module, are to be used directly, without any up-to-date checks.
Note, this is distinct from Swift dependencies that have a textual interface, for which we also detect potential pre-build binary module candidates. Those are still reported in the `details` field of textual Swift dependencies as `prebuiltModuleCandidates`.
This is meant to address a problem that arises on incremental package builds:
A re-scan on an already-built package instead of a placeholder dependency produces a graph that contains a dependency consisting solely of the previously-built swiftmodule.
This is not the behaviour we would like. Instead, we should respect that this is a placeholder dependency and ensure that the dependency graph for that dependency itself captures the fact that a previously-built module exists using compiledModuleCandidates field of the dependency graph.
