[analysis_server_plugin] Support parsed-only analysis for purely syntactic rules

Fixes https://github.com/dart-lang/sdk/issues/64176

Fork of https://dart-review.googlesource.com/c/sdk/+/543580

Allows analyzer plugins to skip element model resolution for
libraries where all enabled rules only require parsed compilation
units (declaring `bool get canUseParsedResult => true;`).

1. Implements `package`, `isInLibDir`, `isInTestDirectory`, and
   `isFeatureEnabled` on `RuleContextWithParsedResults` so syntactic
   rules have access to workspace context.
2. Updates `PluginServer` to evaluate `_canUseParsedResult` across
   library compilation units before resolving libraries.
3. Ensures part files resolve their enclosing library when computing
   parsed diagnostics on change.

Change-Id: I0f9c99ffb8aea9d8921e36657a5cf3b05001cbf3
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/544280
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Commit-Queue: Samuel Rawlins <srawlins@google.com>