Currently, the go/types.Type for a declared type alias
is the same type as the that which it is aliased too.

In Go 1.23, when GODEBUG=gotypesalias=1 is enabled by default,
it is a go/types.Alias instead, which is a unique type node
for aliases that points to the type it is aliased to via go/types.Unalias().
(see golang/go#63223 for more details).

errcheck's tests do not currently fail with GODEBUG=gotypesalias=1,
but that is because the tests themselves do not exercise the problematic situations,
such as the following:

```go
type t struct{}

func (x t) a() error { /* ... */ }

type embedt struct {
	t
}

type embedtalias = embedt

func foo() {
	embedtalias.a()
}
```

With GODEBUG=gotypesalias=1, errcheck will panic when inspecting `embedtalias.a()`
to see if it should be excluded from checking:

```
panic: cannot get Field of a type that is not a struct, got _/home/user/go/src/github.com/kisielk/errcheck/errcheck/testdata/src/a.embedtalias (*types.Alias)

goroutine 2962 [running]:
github.com/kisielk/errcheck/errcheck.getTypeAtFieldIndex({0x7779c0?, 0xc002eceab0?}, 0x4c?)
        /home/user/go/src/github.com/kisielk/errcheck/errcheck/embedded_walker.go:96 +0xf9
github.com/kisielk/errcheck/errcheck.walkThroughEmbeddedInterfaces(0x6b83a0?)
        /home/user/go/src/github.com/kisielk/errcheck/errcheck/embedded_walker.go:53 +0x8e
github.com/kisielk/errcheck/errcheck.(*visitor).namesForExcludeCheck(0xc0025eeff0, 0xc002596140?)
        /home/user/go/src/github.com/kisielk/errcheck/errcheck/errcheck.go:364 +0x165
github.com/kisielk/errcheck/errcheck.(*visitor).excludeCall(0xc0025eeff0, 0x4d?)
        /home/user/go/src/github.com/kisielk/errcheck/errcheck/errcheck.go:403 +0x72
github.com/kisielk/errcheck/errcheck.(*visitor).ignoreCall(0xc0025eeff0, 0xc002596140)
        /home/user/go/src/github.com/kisielk/errcheck/errcheck/errcheck.go:415 +0x2c
github.com/kisielk/errcheck/errcheck.(*visitor).Visit(0xc0025eeff0, {0x776d48?, 0xc0066e08a0})
        /home/user/go/src/github.com/kisielk/errcheck/errcheck/errcheck.go:564 +0x137
```

This change uses `types.Unalias` to follow through this extra level of indirection,
which fixes the panic and ensures the same behavior with both settings.

This change also adds these cases to the test suite, and runs the tests
using both settings of gotypesalias. Without the calls to `types.Unalias`,
the tests fail.