Move WorkspacePackage into analyzer public API

[srawlins]

As part of moving LintRule and it's dependencies into public API (#50986), I see that we expose WorkspacePackage, and intentionally. Lint rules use it to figure out if a file is in public API, or if it's in the lib/ directory, etc.

So I'd like to move WorkspacePackage into public API. Luckily, we can extract a very small subset of the class, into a new class WorkspacePackage, and rename the private impl code WorkspacePackageImpl.

Here's a draft CL: https://dart-review.googlesource.com/c/sdk/+/428564

Here are the new bits in api.txt, quite minimal:

package:analyzer/workspace/workspace.dart:
  WorkspacePackage (class extends Object):
    new (constructor: WorkspacePackage Function())
    canHavePublicApi (getter: bool)
    root (getter: String)
    contains (method: bool Function(Source))
    isInTestDirectory (method: bool Function(File))

[srawlins]

CC @stereotype441 @scheglov