Deduplicate workspace names (#391)

It is possible that the manifest workspaces include the root workspace.
In this case `ConfigurationChie#getAvailableWorkspaceName()` would
return the root name twice. This in turn would result in the root
package being included twice and throw a `ConfigurationError` because of
a duplicate package name.

To address this we deduplicate the workspace names.

Fixes #390

packages/knip/src/ConfigurationChief.ts

@@ -311,9 +311,9 @@ export class ConfigurationChief {
   }
 
   private getAvailableWorkspaceNames() {
-    return [ROOT_WORKSPACE_NAME, ...this.manifestWorkspaces.keys(), ...this.additionalWorkspaceNames].filter(
-      name => !micromatch.isMatch(name, this.ignoredWorkspacePatterns)
-    );
+    return [
+      ...new Set([ROOT_WORKSPACE_NAME, ...this.manifestWorkspaces.keys(), ...this.additionalWorkspaceNames]),
+    ].filter(name => !micromatch.isMatch(name, this.ignoredWorkspacePatterns));
   }
 
   private getAvailableWorkspaceManifests(availableWorkspaceDirs: string[]) {