idl-build doesn't work when default types are used as a type alias

[acheroncrypto]

Using a type alias(#2637) with a default type, e.g.

pub type MyU8 = u8;

will result with

error[E0599]: no associated item named `__anchor_private_full_path` found for type `u8` in the current scope

This is because default the type MyU8 will be treated as a Defined type since it doesn't match any of the default types.

[acheroncrypto]

Fixed by #2824.