Making independent-leaves=true by default in pnpm@4

[zkochan]

As discussed in #1001, having not just the raw package files but also its dependencies in the store would make installations super fast.

However, some packages rely on their location, so it might be a big breaking change to move packages out of project/node_modules to the global store. To make the migration less painful, I suggest moving only packages with no dependencies to the store. We already have a config for that called independent-leaves. This change would not require changes in the shrinkwrap.yaml format.

So in pnpm@2 we would turn on independent-leaves by default. It will make installations approximately 8% faster. In pnpm@3 we would do #1001 and move all packages with their dependencies to the global store. It will require breaking changes in the shrinkwrap.yaml format

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This should work only with packages that don't have to be built

[zkochan]

This may cause issues with packages that use optional peer dependencies. We should support #1486