Use required files for container configuration

[PhilippWendler]

Currently, the tool-info module, the task-definition files, and the benchmark-definition files can all specify required files that are considered relevant for a run. However, BenchExec does not make use of this, only additional modules in contrib/ do. But there is one good use case also for BenchExec: It should make sure that everything that is specified as required is visible in the container regardless of which directory modes are given on the command line.

This would allow users to specify something like --hidden-dir /home (which is good for isolation and reproducibility) without having to worry to manually include the tool directory and the directories with the input files. This is especially useful for users on non-Ubuntu systems where overlayfs cannot be used. In a future major version we could even make --read-only-dir / --hidden-dir /home the default if this feature turns out useful in practice and thus make BenchExec usable on non-Ubuntu systems without additional parameters.