Prevent adding non-existing package

[shuuji3]

Issue

We can add non-existing packages or packages with invalid name, resulting with meaningless comparison with no data 😅

("存在しないパッケージ" means "non-existing package" in Japanese)

Exapected behavior

Users can add only existing packages to the comparison table.

[MatteoGabriele]

@iiio2 double PR moment 😆