Idea: separate npm/yarn/pnpm/lerna package managers

[rarkins]

The existing "npm" manager in Renovate is really complex, partly due to its combination of npm/pnpm/yarn/lerna. I wonder if it would be feasible to separate them into separate package managers, albeit with a lot of shared code.

npm - if package-lock.json or npm-shrinkwrap.json are present, or no other managers are present (i.e. it's the default)
yarn - if yarn.lock is present. But not if there's lerna?
pnpm - look for pnpm's lock files
lerna - does it take precedence over yarn workspaces, or does yarn workspaces mean we can ignore lerna?

[viceice]

superseeded by #5607