[lldb-vscode] improve modules request
lldb-vsdode was communicating the list of modules to the IDE with events, which in practice ended up having some drawbacks
- when debugging large targets, the number of these events were easily 10k, which polluted the messages being transmitted, which caused the following: a harder time debugging the messages, a lag after terminated the process because of these messages being processes (this could easily take several seconds). The latter was specially bad, as users were complaining about it even when they didn't check the modules view.
- these events were rarely used, as users only check the modules view when something is wrong and they try to debug things.
After getting some feedback from users, we realized that it's better to not used events but make this simply a request and is triggered by users whenever they needed.
This diff achieves that and does some small clean up in the existing code.
Differential Revision: https://reviews.llvm.org/D94033