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

[stereotype441]

I like the idea of finding the minimal subset of WorkspacePackage that needs to be public and just exposing that. Someone with more familiarity with the class itself should probably review your CL, but I'm on board with the general idea!