[IMPROVEMENT] Allow selection of package sources before searching for packages

### Please confirm these before moving forward.

- [x] I have searched for my feature proposal and have not found a work-in-progress/duplicate/resolved/discarded issue.
- [x] This improvement refers to an existing feature. If you want to suggest a new feature, please use [this template](https://github.com/Devolutions/UniGetUI/issues/new?labels=new-feature&projects=&template=feature-request.yml&title=%5BFEATURE+REQUEST%5D+%28Enter+your+description+here%29).
- [x] This improvement is not a bug. If you want to report a bug, please use [this template](https://github.com/Devolutions/UniGetUI/issues/new?labels=bug&projects=&template=bug-issue.yml&title=%5BBUG%5D+%28Enter+your+description+here%29).

### Describe the improvement

When searching for a tool on winget, I really don't want to see packages from all my package managers and it clutters the list, so letting me select just winget from the start would improve the experience.

### Describe how this improvement could help users

Only search the sources they want a package from.

Start by tracing the package search flow and the interface where package-manager sources are selected. Implement source selection before searching so users can choose only the desired source, such as winget, and verify that results exclude unselected package managers.

Written by the indexing model from the issue text.