Revert "Support multiple packages in a PubWorkspace."

This reverts commit d032598d26da3a6d1734604336b604a177ecd29d.

Reason for revert: SDK build is broken if the checkout path includes "build" as a path segment.

Original change's description:
> Support multiple packages in a PubWorkspace.
>
> A PubWorkspace is now
> - rooted at package config, or the outermost pubspec if there is no package config
> - packages are identified by pubspec
> - all packages are considered to be package build packages
>
> Change-Id: Id1552ab584ccedf60a482b93b375d1c4c7da6796
> Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/337607
> Commit-Queue: Keerti Parthasarathy <keertip@google.com>
> Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
> Reviewed-by: Samuel Rawlins <srawlins@google.com>

Change-Id: I2ecda907b8bb6164c7cc5446aceff1f168810341
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/338560
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Commit-Queue: Martin Kustermann <kustermann@google.com>
Auto-Submit: William Hesse <whesse@google.com>
Reviewed-by: Martin Kustermann <kustermann@google.com>