[API summary] Consider publicly exported names to be part of public API.

Changes the logic for deciding whether to output details about a top
level element. Previously, details would be output if the library
containing the element's declaration was in `lib` but not
`lib/src`. This led to a bug: if a top level element was declared in
`lib/src` but exported by an `export` directive in `lib`, no details
would be output, and the API summary would just show `(non-public)`
after the exported name.

This bug was mostly benign because we were working around it with an
analyzer-specific hack: when analyzing the `analyzer` package, top
level elements with an annotation of type `AnalyzerPublicApi` would
have their details output regardless of where they were declared. But
it wasn't completely benign: the tool was failing to output details of
`DartDocumentLinkVisitor` and `DocumentLink` (from
`package:analyzer_plugin`), as well as `PackageBuilder` (from
`package:analyzer_testing`).

The new logic is: details are output if the element appears in the
export namespace of any library in `lib` but not `lib/src`. I've
re-run the API summary tool so the `api.txt` files in
`package:analyzer_plugin` and `package:analyzer_testing` now include
the details they were missing.

The analyzer-specific hack is left in place, though, because there are
some analyzer classes that aren't exported, but still considered part
of the analyzer public API. In a follow-up CL, I will make the API
summary tool extensible so that this analyzer-specific logic can be
injected by the analyzer when generating its `api.txt` file, and it
won't pollute the incipient `api_summary` tool.

Change-Id: I6a6a69641d656caa4f8e6361557c13fa7485e422
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/482440
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>