Importing repos from GHE doesn't work

With the introduction of caching in 8.6. to Repository.exists? to improve performance, importing repos from GHE doesn't work. First, Projects::ImportService import the repository, and then call Gitlab::GithubImport::Importer to import the data (issues, pull requests, and wiki). Projects::ImportService should expires the cache for this repository before the data import start. For example, when loading the merge request content Repository#exists? gets called, which could returns false when it should be true.

/cc @yorickpeterse @balameb