Some tests used a fake vendor directory placed inside _fixtures to
import some support packages.
In go.mod mode vendor directory are only supported on the root of the
project, which breaks some of our tests.
Since vendor directories outside the root of the project are so rare
anyway it's possible that a future version of go will stop supporting
it even in GOPATH mode.
Also it was weird and unnecessary in the first place anyawy.
If the last entry of the package path contains a '.' the corresponding
DIEs for its types will replace the '.' character with '%2e'. We must
do the same when resolving the package path of the concrete type of an
interface variable.
Fixes#1137
