Build Command Changes

A PR in early 2020 introduced some important changes to Fabric's build commands which are detailed below. The Build Command documentation has been updated to reflect these changes.

Monorepo Build Command	Prereqs	Important Notes	Description
`yarn build`		Now TypeScript only. No longer runs bundle, lint or test. No longer production build by default.	Generates output using TypeScript.
`yarn buildci`		No change. This command approximates PR build steps and is useful for validating your build locally.	Reproduction of PR build steps: build, lint, test, bundle.
`yarn buildto`		Now TypeScript only. No longer runs bundle, lint or test. No longer production build by default.	Builds to specified target package.
`yarn buildfast`		Removed (redundant with `yarn build` and `yarn buildto`)
`yarn bundle`	build	New	Runs webpack. Webpack configs use lib output as entry point, requiring build as prereq.
`yarn clean`		New	Cleans package output.
`yarn lint`	build	New	Runs lint and lint-imports tasks. lint-imports requires build artifacts to detect that physical file exist for imports from 'lib'.
`yarn test`	build	New (replaces `yarn validate`)	Runs jest. Build artifacts required for dependencies of packages under test.
`yarn validate`		Removed (replaced by `yarn test`)

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