Don't check non ast.Ident expressions for been Gomega variables.

Make more accurate check for the Gomega types (struct or interface).
check the alias value instead of the alias.

ginkgolinter fails to recognized ginkgo.SpecContext as a valid context in
Eventually, and so wrongly identify it as the actual value, instead of
the following function parameter.

This PR fixes the issue and now this scenarion works

```go
It("should work", func(ctx SpecContext) {
	Eventually(func() error {
		return fmt.Errorf("foo")
	}).WithContext(ctx).Should(MatchError("foo"))
}, SpecTimeout(time.Second))
```