Use the name specified by compile unit attribute DW_AT_go_package_name,
introduced in Go 1.13, to map package names to package paths, instead of
trying to deduce it from names of types.
Also use this mapping for resolving global variables and function
expressions.
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.