Prevent build stalling with overlapping projections (#2602)

## Motivation and Context

Smithy CLI runs projections concurrently, for some currently unknown
reason this causes the build to stall.

## Description

- Switch `defaultRustMetadata` in `BaseSymbolMetadataProvider` from a
`val` to a `fun`.
- Remove unused `RustType::TestModule` companion object.

## Notes

A deadlock occurs when the client/server plugins both attempt to grab
file locks. While this fixes this deadlock it is unknown whether there
still exist conditions where it can happen.

## Testing

1. Checkout
crisidev/smithy-rs-pokemon-service@d276bb9,
run `./gradlew assemble`, and observe the build halting.
2. Run the following commands to switch to this branch:
```bash
git submodule update --init --recursive --remote
cd smithy-rs
git checkout harryb/remove-build-deadlock
cd ..
./gradlew assemble
```
3. Observe the build succeeding.

Author: hlbarber

codegen-core/src/main/kotlin/software/amazon/smithy/rust/codegen/core/rustlang/RustType.kt

@@ -422,15 +422,6 @@ data class RustMetadata(
     fun hasDebugDerive(): Boolean {
         return derives.contains(RuntimeType.Debug)
     }
-
-    companion object {
-        val TestModule = RustMetadata(
-            visibility = Visibility.PRIVATE,
-            additionalAttributes = listOf(
-                Attribute.CfgTest,
-            ),
-        )
-    }
 }
 
 data class Argument(val argument: String, val value: String, val type: String)

codegen-core/src/main/kotlin/software/amazon/smithy/rust/codegen/core/smithy/SymbolMetadataProvider.kt

@@ -137,13 +137,13 @@ class BaseSymbolMetadataProvider(
 
     // Only the server subproject uses these, so we provide a sane and conservative default implementation here so that
     // the rest of symbol metadata providers can just delegate to it.
-    private val defaultRustMetadata = RustMetadata(visibility = Visibility.PRIVATE)
+    private fun defaultRustMetadata() = RustMetadata(visibility = Visibility.PRIVATE)
 
-    override fun listMeta(listShape: ListShape) = defaultRustMetadata
-    override fun mapMeta(mapShape: MapShape) = defaultRustMetadata
-    override fun stringMeta(stringShape: StringShape) = defaultRustMetadata
-    override fun numberMeta(numberShape: NumberShape) = defaultRustMetadata
-    override fun blobMeta(blobShape: BlobShape) = defaultRustMetadata
+    override fun listMeta(listShape: ListShape) = defaultRustMetadata()
+    override fun mapMeta(mapShape: MapShape) = defaultRustMetadata()
+    override fun stringMeta(stringShape: StringShape) = defaultRustMetadata()
+    override fun numberMeta(numberShape: NumberShape) = defaultRustMetadata()
+    override fun blobMeta(blobShape: BlobShape) = defaultRustMetadata()
 }
 
 private const val META_KEY = "meta"