Add CLI switch to disable use of file locking

[a-ludi]

System information

• DUB version 1.15.0
• Lustre file system without file locking

Bug Description

It is currently not possible to use DUB if the package cache is located on a file system that does not implement file locking.

Expected Behavior

In order to support these file systems, a CLI switch --no-lock should be added which disables file locking but not the lock file itself. When the switch is in effect DUB will still check if the lock file is present or create it otherwise. Even though this might lead to race conditions, it is the best options if file locking is not supported.

There might be a better name for the switch because --no-lock suggests not even a lock file being created/checked.

An alternative to the CLI switch would be to dynamically check if file locking is supported and decide for the safest locking strategy available.