Migrate .codechecker folder from old versions

[Discookie]

In previous versions, the folder structure of .codechecker looked something like:

.codechecker
  - compile_commands.json
  - metadata.json
  - xyz.plist
  - ...

With CodeChecker 1.3.0 (commit 36448fb), the CodeChecker output was moved into a subfolder:

.codechecker
  - compile_commands.json
  - reports
    - metadata.json
    - xyz.plist
    - ...

However, there was no handling or migration logic implemented at the time, so now there's CodeChecker metadata in both the root folder and the reports folder. The original reports folder should be cleaned up or migrated into the subfolder.