Support source-context resolution from multiple files

[vaind]

Historically, only source-bundles contained sources for source-context. Nowadays, you can also have them in Portable PDBs, whether as embedded, or as links. In the future, other formats may follow.

In order to resolve source context as best as we can, we should not limit resolution to a single debug file that reports has_sources() = true but instead try to match each of them until a match is found. Alternatively, they should be combined. This resolution could be cached if needed.