Add checks for conflicts for aliases

[Mingun]

• Check that alias is not the same as name of other field / variant (it still can be the name of owning field / variant)
• Check that aliases are unique, i. e. two different fields / variants does not use the same alias

It would be better, if error points to corresponding alias, but currently alias spans are not stored anywhere, so I left this to another PR.

Fixes #2308, fixes #2551