Re-introspection should not remove relations when foreign keys are not allowed

[pimeys]

Introspection engine should not overwrite relation fields in the PSL, if the datasource has the referential integrity set to happen in Prisma.

Currently if we have the following data model:

datasource db {
  provider             = "mysql"
  url                  = env("DATABASE_URL")
  referentialIntegrity = "prisma"
}

generator client {
  provider        = "prisma-client-js"
  previewFeatures = ["referentialIntegrity"]
}

model A {
  id  Int @id @default(autoincrement())
  bId Int
  b B @relation(fields: [bId], references: [id], onDelete: Restrict, onUpdate: SetNull)
}

model B {
  id Int @id @default(autoincrement())
  as A[]
}

When we migrate this to the database, the foreign key constraints are not written due to the settings in the datasource. When we again run db pull, the introspection engine will overwrite the data model, removing all relations due to not finding any foreign keys:

datasource db {
  provider             = "mysql"
  url                  = env("DATABASE_URL")
  referentialIntegrity = "prisma"
}

generator client {
  provider        = "prisma-client-js"
  previewFeatures = ["referentialIntegrity"]
}

model A {
  id  Int @id @default(autoincrement())
  bId Int
}

model B {
  id Int @id @default(autoincrement())
}

We should make the re-introspection to not do anything to the relations, if we do not use foreign keys.