Check for go package directory existence before accessing

[jonabc]

Fixes a crash seen when go list tells licensed that a package has a directory that doesn't exist. This was seen when referencing a repo with multiple module while also using GOFLAGS="-mod=vendor".

In this case go list was assuming that all dependent packages used would live in the vendor folder, which is not true for other modules defined in the repo. In go < 1.14 this causes go list to fail entirely and in go 1.14 the reference to a directory that doesn't exist causes licensed to crash by trying to Dir.chdir to the non-existent repo. The fix here checks for the existence of the repo before trying to change to its directory.

/cc @vroldanbet @macripps