aspellWithDicts: use a single env

In c0cf1960 the function
`aspellWithDicts` was introduced, that allows to build a derivation
consisting of aspell and specified dictionaries. In
96457d26 a fix was included to properly
find the dictionaries.

Issue #29429 describes that, while the current method works for the
aspell binary, it does not in case of the API.

This commit rewrites the wrapper into a single derivation, create a
single tree of symbolic references to both the binary and the
dictionaries so that its possible to find the dictionaries with the API.
Furthermore, the binary is wrapped so it can still find the dictionaries
as well.