[beta] AO. Issue 64549. Make included exclude patterns relative to the root options file.

Issue description: Since SDK 3.14 beta (analyzer 14.2.0), each
`analyzer: exclude:` pattern is relative to the folder of the options
file that declares it. So patterns from an included options file, such
as `include: package:my_lints/options.yaml`, or a workspace root
`include: ../../analysis_options.yaml`, no longer exclude files of the
including package. Generated files (`*.g.dart`, `build/**`) are
analyzed, reporting diagnostics and slowing analysis. Affects `dart
analyze`, the analysis server (IDEs), and all platforms. SDK 3.13 is not
affected.

What is the fix: Restore the behavior of SDK 3.13: every exclude
pattern, including patterns from included files, is relative to the
folder of the `analysis_options.yaml` at the start of the `include:`
chain.

Why cherry-pick: This is a regression from 3.13 in a very common setup:
shared lint packages and monorepo root options files commonly declare
`exclude:` for generated code. Without the fix, 3.14 users see errors
and warnings in generated and excluded files, and must duplicate exclude
patterns into every package.

Risk: Low. The change is a few lines in
`_ContextLocator._getExcludedGlobs`, restores the 3.13 behavior, has
landed on master with tests, and is covered by new tests that fail
without it.

Issue link(s): https://github.com/dart-lang/sdk/issues/64549

Bug: https://github.com/dart-lang/sdk/issues/64549
Change-Id: I19e1aef1f73393e1a7a814fc9501c3c1848c98e7
Cherry-pick: https://dart-review.googlesource.com/c/sdk/+/562741
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/563062
Reviewed-by: Alexander Thomas <athom@google.com>
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>
Reviewed-by: Paul Berry <paulberry@google.com>