Rethink on how the registry is setup

[deirn]

Currently call to mason-ensure, in addition to PATH setup, will also retrieve the registry.
This is totally unnecessary if you don't plan to install any package in current session, and will just waste memory for the registry.

Some idea:

• Change mason-ensure to just for PATH setup and only load registry when mason-install et al is called.
• Also clean the registry after you done installing.

I'm not sure how it will affect the performance.