Enable strict concurrency (#182)

### Motivation:

Catch potential data races at build time.

### Modifications:

- Enabled strict concurrency checking in the Package.swift and removed
it from the CI flags.

### Result:

Fewer potential data races can sneak in.

### Test Plan

Ran tests locally, did not see any concurrency warnings or errors.

Author: czechboy0

.github/workflows/main.yml

@@ -11,8 +11,8 @@ jobs:
     name: Unit tests
     uses: ./.github/workflows/unit_tests.yml
     with:
-      linux_5_9_arguments_override: "--explicit-target-dependency-import-check error -Xswiftc -strict-concurrency=complete"
-      linux_5_10_arguments_override: "--explicit-target-dependency-import-check error -Xswiftc -strict-concurrency=complete"
+      linux_5_9_arguments_override: "--explicit-target-dependency-import-check error"
+      linux_5_10_arguments_override: "--explicit-target-dependency-import-check error"
       linux_6_0_arguments_override: "--explicit-target-dependency-import-check error"
       linux_nightly_6_0_arguments_override: "--explicit-target-dependency-import-check error"
       linux_nightly_main_arguments_override: "--explicit-target-dependency-import-check error"

.github/workflows/pull_request.yml

@@ -17,8 +17,8 @@ jobs:
     name: Unit tests
     uses: ./.github/workflows/unit_tests.yml
     with:
-      linux_5_9_arguments_override: "--explicit-target-dependency-import-check error -Xswiftc -strict-concurrency=complete"
-      linux_5_10_arguments_override: "--explicit-target-dependency-import-check error -Xswiftc -strict-concurrency=complete"
+      linux_5_9_arguments_override: "--explicit-target-dependency-import-check error"
+      linux_5_10_arguments_override: "--explicit-target-dependency-import-check error"
       linux_6_0_arguments_override: "--explicit-target-dependency-import-check error"
       linux_nightly_6_0_arguments_override: "--explicit-target-dependency-import-check error"
       linux_nightly_main_arguments_override: "--explicit-target-dependency-import-check error"

Package.swift

@@ -111,3 +111,9 @@ let package = Package(
         ),
     ]
 )
+
+for target in package.targets {
+    var settings = target.swiftSettings ?? []
+    settings.append(.enableExperimentalFeature("StrictConcurrency=complete"))
+    target.swiftSettings = settings
+}