Allow overriding of package config values by matching version

[josephschmitt]

For example:

package "pnpm" {
  bins = {
    "pnpm": "bin/pnpm.cjs",
    "pnpx": "bin/pnpx.cjs",
  }
}

package pnpm "<6" {
  bins = {
    "pnpm": "bin/pnpm.js",
    "pnpx": "bin/pnpx.js",
  }
}