Resolve using declared before the file namespace from the global namespace

[timotheeguerin]

Writing a using above a file-level namespace silently changed what it meant. Because the using was resolved as if it lived inside the file namespace, this failed:

using TypeSpec.Http;
namespace _Specs_.TypeSpec.Foo;

error invalid-ref: Namespace _Specs_.TypeSpec doesn't have member Http

TypeSpec bound to _Specs_.TypeSpec instead of the global TypeSpec, even though the using is written before the namespace declaration. Switching to a block namespace made it work, which is confusing.

Our namespace system is modelled on C#, and C# does not have this problem: a using before a file-scoped namespace is compilation-unit level and resolves from the global namespace; a using after it lives inside the namespace and resolves relatively. This PR adopts the same rule.

using Common.Models;              // resolves the global `Common`
namespace MyOrg.Common.Service;

namespace MyOrg.Service;
using Models;                     // unchanged: resolves `MyOrg.Models`

Breaking change

A relative name in a using written above the file namespace now needs to be fully qualified, otherwise it reports Unknown identifier.

Follow-ups

• Suggest the fully qualified name when a using before the file namespace fails to resolve #11560: a dedicated diagnostic with a quick fix suggesting the fully qualified name for the breaking case above.
• Referencing a shadowed global namespace from the file body still needs an escape hatch — C# has global::, TypeSpec has no equivalent.

Fixes #1840

[pkg-pr-new]

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npm i https://pkg.pr.new/@typespec/compiler@11552

commit: a12422e

[github-actions]

All changed packages have been documented.

• ✅ @typespec/compiler

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@typespec/compiler - breaking ✏️

using statements declared before a file-level(blockless) namespace are now resolved from the global namespace instead of the file namespace, matching C#.,> ,> tsp,> using TypeSpec.Http; // Now resolves the global `TypeSpec` namespace instead of `_Specs_.TypeSpec`,> namespace _Specs_.TypeSpec.Foo;,> ,> ,> A using declared after the file namespace, or inside a namespace block, is unchanged and still resolves relative to that namespace. Code relying on a relative name in a using written above the file namespace must now use the fully qualified name.,> ,> tsp,> namespace MyOrg.Service;,> using Models; // Still resolves to `MyOrg.Models`,>

[azure-sdk-automation]

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