fix: print information message when package-manager is set
fix: print information message when package-manager is set

Currently when calling `tns package-manager set <valid value>` you do not see any output or information if the operation is successful. Add such message and clean the code from hardcoded strings.

NativeScript/nativescript-cli,NativeScript/nativescript-cli,NativeScript/nativescript-cli,NativeScript/nativescript-cli