fix: detect colliding go imports at emit time

[ivov]

Adds a diagnostic when two Go packages emitted into the same Go file default to the same package name. Previously the user saw a raw go build failure with redeclared in this block and no hint where it came from.

import "go:entgo.io/ent/dialect/sql"

fn main() {
  let driver = sql.Open(dialect.Postgres, dsn)?
}

Compiling this now reports the collision between entgo.io/ent/dialect/sql and database/sql (pulled in transitively by ent's typedefs) and suggests import entsql "go:entgo.io/ent/dialect/sql".

Complements #212 and #217, which alias colliding imports inside generated typedefs. This catches the case those cannot see, i.e. a user's direct import colliding with a transitive one.

[ivov]

Released in lisette-v0.1.23