Scanner: Avoid returning irrelevant issues

[fviernau]

The scanner might run into an issue which related only to a single file.
If an issue related to a file which not within the set of files resulting from filtering the scan result by VCS path, then
the issue should also be filtered out. This happens e.g. for TIMEOUT issues.

In this context, it makes sense to consider enabling the path excludes to silence timeouts as well.

[fviernau]

Thinking about solving this by either:

1. extending ScanSummary to have a dedicated property for issues which are specific to a single file. Maybe just a
   map filePath -> issue ?
2. Use polymorphism with the Issue class, so that it has additional properties.
   A subtype could be ScanTimeoutIssue with the property filePath.
3. Just add list of files which could not be scanned: `ScanSummary.fileScanErrors: List

[mnonnenmacher]

Do we get the affected file for such issues from the ScanCode result model or would we have to parse the error messages to try to find out if an issue belongs to a single file?

[sschuberth]

2. Use polymorphism with the Issue class, so that it has additional properties.

FYI, this roughly goes into the direction of #4393.