[v2.0.0] Re design of the plugin

[yoannmoinet]

What and why?

This PR redesigns how this plugin is implemented to facilitate the composition we'll need to do later on.

How?

We move to a packages approach using yarn workspaces.

• @datadog/build-plugins-assets | ./packages/assets: for all the static assets we may use.
• @datadog/build-plugins-core | ./packages/core: for everything that is re-used somewhere else.
• @datadog/esbuild-plugin | ./packages/esbuild-plugin: for the esbuild plugin.
• @datadog/build-plugins-tests | ./packages/tests: for all the tests we have.
  • project-esbuild | ./packages/tests/mocks/projects/esbuild: for a mock project built with esbuild.
  • project-webpack4 | ./packages/tests/mocks/projects/webpack4: for a mock project build with webpack 4.
  • project-webpack5 | ./packages/tests/mocks/projects/webpack5: for a mock project build with webpack 5.
• @datadog/build-plugins-tools | ./packages/tools: for the various tools we use on the infra side, aka a CLI.
• @datadog/webpack-plugin | ./packages/webpack-plugin: for the webpack plugin.

Everything else in the PR is about updating dependencies and make everything work together.