Step in https://github.com/paritytech/polkadot-sdk/issues/171

This PR removes the need to specify `as [disambiguation_path]` for cases
where the trait definition resides within the same scope as default impl
path.

For example, in the following macro invocation
```rust
#[derive_impl(frame_system::config_preludes::TestDefaultConfig as frame_system::DefaultConfig)]
impl frame_system::Config for Runtime {
   ...
}
```
the trait `DefaultConfig` lies within the `frame_system` scope and
`TestDefaultConfig` impls the `DefaultConfig` trait. Using this
information, we can compute the disambiguation path internally, thus
removing the need of an explicit specification.

In cases where the trait lies outside this scope, we would still need to
specify it explicitly, but this should take care of most (if not all)
uses of `derive_impl` within FRAME's context.