This PR makes two changes to the `lambda-at-edge` builder to better
support Yarn v2 projects when using the `serverless-trace` target.

1. Adds a `baseDir` build option to specify the base directory to search
   for `node_modules`. Currently, the builder sets this to `process.cwd()`,
   but Yarn v2 often hoists dependencies across multiple workspaces so
   that they can be shared. Without this change, all dependencies from
   ancestor directories are omitted, leading to import errors at runtime.
2. Adds a `resolve` build option which allows projects to specify their
   own custom resolvers, such as when supporting Yarn v2's PnP mode.
   This leverages the resolver hook added in vercel/nft#153, which also
   required upgrading from `@zeit/node-file-trace` to the latest version
   of `@vercel/nft`. Note that even after this change, Yarn v2's PnP mode
   is still incompatible with the builder since the builder ends up
   collapsing all of the PnP dependencies into a single `node_modules`
   directory. This causes multiple versions of a package to clobber one
   another.