Replace `ethers` umbrella package with submodules (`@ethersproject/*`)

[Old ticket](https://github.com/metamask/metamask-extension/issues/15022)

We currently include the whole `ethers` library in our dependency tree, but it should be possible to just require and import only the packages we need.

See the readme here for an example: https://github.com/ethers-io/ethers.js/tree/master/packages/contracts

The task is to replace all instances where we import all of ethers with the specific package that is needed, and then to remove the general package from package.json. We should compare the build size before and after this change to confirm it has the desired affect.