Simplify linking step after `indo add` completes install

[aleclarson]

In the case of Yarn and PNPM (and maybe NPM?), existing symlinks are preserved when a new dependency is installed, so we only need to check if the newly installed packages can be linked, instead of all packages.

[aleclarson]

Same goes for indo link

Progress is tracked in https://github.com/alloc/indo/tree/wip/efficient-link