redo recursive directory matching with logging

[brettfo]

When a job starts a PowerShell script first scans for relevant global.json files to install the appropriate SDKs, but when this fails it's really difficult to diagnose.

This PR uses a regex-based approach to matching recursive subdirectories and logs absolutely everything it does so failures to find or install SDKs can be debugged.

[Copilot]

Pull request overview

This PR updates the NuGet updater’s PowerShell SDK discovery path to use regex-based recursive directory matching and add diagnostic logging around global.json discovery and SDK/targeting-pack installation.

Changes:

• Adds gated PowerShell logging via DEPENDABOT_LOG_MESSAGES.
• Reworks global.json discovery to enumerate repo directories and match configured update directories with generated regexes.
• Adds test fixtures and cases for deep and recursive global.json discovery.

Show a summary per file

File	Description
nuget/updater/common.ps1	Adds logging and regex-based directory discovery for SDK install decisions.
nuget/updater/main.ps1	Enables diagnostic logging during update execution.
nuget/updater/test.ps1	Adds test coverage for deep and recursive wildcard discovery cases.
nuget/updater/test-data/global-json-discovery-deep/src/client/global.json	Fixture for a nested global.json.
nuget/updater/test-data/global-json-discovery-recursive-wildcard-skip-root/global.json	Fixture for root-level SDK discovery during recursive matching.
nuget/updater/test-data/global-json-discovery-recursive-wildcard-skip-root/src/global.json	Fixture for matched subdirectory SDK discovery.

Copilot's findings

• Files reviewed: 6/6 changed files
• Comments generated: 2