RFC(resolvers): new import syntax of mappers (mostly namespaces but not only!)

[kamilkisiela]

Default type

./my/module#default
vs
./my/module

Default type (with alias)

./my/module#default as Alias
vs
./my/module as Alias

Type

./my/module#Foo
vs
./my/module#Foo

Type (with alias)

./my/module#Foo as FooModel
vs
./my/module#Foo as FooModel

Namespace

./my/module#Models#Foo
vs
./my/module#Models.Foo

Namespace (with alias)

./my/module#Models#Foo as FooModel
vs
./my/module#Models.Foo as FooModel

[kamilkisiela]

@dotansimha #3501

It doesn't support aliases in namespaces and defaults imported as ./my/module (instead of ./my/module#default). Aliases in namespaces would require an additional type generation like type Alias = Namespace.Type and defaults without #default means we can't really decide if the mapper comes from external module or it's a string, we could check it based on . or / but then what if someone has User: module in their codegen.yml. We could enforce relative paths with ./ but then it's a breaking change.

But I think it's fine to let people use a nicer syntax even today.