This change refactors the top-level dependency scanning flow to follow the following procedure:
Scan():
1. From the source target under scan, query all imported module identifiers for a *Swift* module. Leave unresolved identifiers unresolved. Proceed transitively to build a *Swift* module dependency graph.
2. Take every unresolved import identifier in the graph from (1) and, assuming that it must be a Clang module, dispatch all of them to be queried in-parallel by the scanner's worker pool.
3. Resolve bridging header Clang module dpendencies
4. Resolve all Swift overlay dependencies, relying on all Clang modules collected in (2) and (3)
5. For the source target under scan, use all of the above discovered module dependencies to resolve all cross-import overlay dependencies
Add support for cross import modules by ingesting swiftoverlay files for
the cross import into CAS file system.
The long-term better fix will be just passing the cross import
information from scanner to swift-frontend so frontend doesn't need to
read overlay files again to figure out the cross import module.
rdar://123839248
Otherwise they may have module dependencies of their own which will not be detected by the scanner and included in the list of explicit inputs for compilation.
'ModuleDependencyScanner' maintains a Thread Pool along with a pool of workers
which are capable of executing a filesystem lookup of a named module dependency.
When resolving imports of a given Swift module, each import's resolution
operation can be issued asunchronously.
