Detect invalid package locales and warn the user

[baronfel]

Fixes #26711

There's a long discussion but the gist is that we can detect situations where locale codes are mismatched and tell the user about it. We can even correct those that can be normalized via the BCL culture lookups.

Open questions:

• what should we do when an invalid culture is found? We have two options I see immediately: include it with a warning, or exclude it. Is there a use case for invalid culture identifiers?
  • decided to allow invalid locale identifiers - ICU data can change, and may not be consistent between build machine and execution machine.
• what code should we choose?
  • took the next two open numbers
• need to think about the messages a bit more - is the packageName@packageId format useful?
  • spoke with Jon Douglas and got the normal nuget output format for package names and versions.

[baronfel]

I'm only marking this for review to see if the tests run - there are more open questions

[baronfel]

Alright - there's only one major question we should answer - what do we do with assets that are found using completely unknown locale identifiers? We can drop them (currently implemented) or include them. Should we be forwards-compatible for locale identifiers? How often does this data change?

[baronfel]

Country codes change based on an IANA registry: https://www.iana.org/assignments/language-subtag-registry/language-subtag-registry

[dsplaisted]

Overall looks good. Please make sure to do some end to end testing before merging (Ie building projects that reference packages that we know have non-normalized locales, as well as packages that have correct locales).

[baronfel]

Manual testing shows it working as expected:

C:\Users\chethusk\OSS\dotnet-sdk\artifacts\bin\redist\Debug\dotnet\sdk\7.0.100-dev\Sdks\Microsoft.NET.Sdk\targets\Microsoft.PackageDependencyResolution.targets(267,5): warning NETSDK1187: Package JavaScriptEngineSwitcher.Core 3.3.0 has a resource with the locale 'ru-ru'. This locale has been normalized to the standard format 'ru-RU' to prevent casing issues in the build. Consider notifying the package author about this casing issue. [C:\Users\chethusk\OSS\dotnet-sdk\blah\blah.csproj]

[baronfel]

One thing I'd like to call out - we've been fairly conservative about introducing new warnings/errors purely on updating the SDK. It typically requires updating a Warning or Analysis level (which by default is tied to TFM), which is a user-driven action. That falls over a bit here because this bug can impact anyone on any TFM, but I just wanted to call out that we are breaking our own rule a bit here.