Make poetry discovery faster

[karrtikr]

For #8372 Closes https://github.com/microsoft/vscode-python-internalbacklog/issues/125

• Add caching to command output in discovery component. When the extension is starting up sometimes the same commands are run again and again, hence cache for more duration during startup.
• Poetry commands can be slow and so can be bottleneck to overall discovery time. So trigger command execution as soon as possible. To do that we need to ensure the operations before the command are performed synchronously.
• Get python executables using getInterpreterPathFromDir instead of finding interpreters in all directories.