Move KeychainWrapper extensions into FxAClient module.

The FxAClient Swift module is the only thing that uses
the KeychainWrapper extensions, so let's make them live
inline in that codebase. This will simplify eventual
migration to publishing as separate Swift modules.

megazords/ios/MozillaAppServices.xcodeproj/project.pbxproj

@@ -488,6 +488,7 @@
 			children = (
 				99FAA19025E60D2C001E2231 /* Generated */,
 				CE0A9AB224E4A25C00914A16 /* fxa_client.udl */,
+				CE1B09A2231863D7006226E1 /* KeychainWrapper+.swift */,
 				CEDDBC8D23DB4CD600CFF5AA /* PersistedFirefoxAccount.swift */,
 				99FAA19E25E65CA5001E2231 /* FxAccountOAuth.swift */,
 				CE1445AB23D6058B00B1E808 /* FxAccountConfig.swift */,
@@ -543,20 +544,11 @@
 			path = RustLog;
 			sourceTree = "<group>";
 		};
-		CE1B099D231862D5006226E1 /* Extensions */ = {
-			isa = PBXGroup;
-			children = (
-				CE1B09A2231863D7006226E1 /* KeychainWrapper+.swift */,
-			);
-			path = Extensions;
-			sourceTree = "<group>";
-		};
 		CE9D201620914D0D00F1C8FA = {
 			isa = PBXGroup;
 			children = (
 				C852EEF2220A3C6800A6E79A /* MozillaAppServices.h */,
 				993D6D4D26B11D9E0075A820 /* Sync15 */,
-				CE1B099D231862D5006226E1 /* Extensions */,
 				CD85A44822361E880099BFA9 /* Places */,
 				CDC21B11221DCE3700AA71E5 /* RustLog */,
 				CE9FFA11242D4E5A0011029E /* Viaduct */,