[release/6.0] Work around dotnet/msbuild#3274 (#39293)

* [release/6.0] Work around dotnet/msbuild#3274
- `cherry-pick` of e2707e1e

Work around dotnet/msbuild#3274
- #38547
- handle a ApiDescription.Client code generator that uses `%(OutputPath)` as a directory
  - see also <https://stackoverflow.com/questions/48868060/can-a-task-itemgroup-glob-files>
- add tests for Microsoft.Extensions.ApiDescription.Client.targets
  - make `TemporaryCSharpProject` slightly extensible
  - allow project additions to `TemporaryDirectory` after `Create()`

nits:
- add comment about batching w/ a code generator that uses `%(OutputPath)` as a directory
- fix `%(SourceDocument)` metadata
  - primarily for back-tracing in detailed / binary logs
- remove useless `%(OutputPathExtension)` metadata; use `%(Extension)`
- remove useless `ConsoleClient` test asset project

Handle symbolic links in `TemporaryDirectory`
- e.g. on macOS /var/folders resolves to /private/var/folders

* !fixup! Align indentation per review comments