[R] install_github broken for dependencies on packages previously installed from private github repo.

[Mark Mendel]

When installing from a private repo, fails if the package depends on
another package from the private repo--even if its already installed.
Example makes this clear

```R
install_github('MyAcct/Preinstalled.Package', auth_token = 'x')
# works
install_github('MyAcct/My.Package', auth_token = 'x', upgrade = FALSE)
# Fails if My.Package depends on Preinstalled.Package -- see below
```
I get the following errors.
```
Downloading GitHub repo MyAcct/My.Package using master
Error: HTTP error 404.
  Not Found

  Did you spell the repo owner (`MyAcct`) and repo name
(`Preinstalled.Package`) correctly?
  - If spelling is correct, check that you have the required permissions to
access the repo.
```

Why is it even checking the repo, since the package is already installed?

This used to work under the old devtools::install_github implementation,
but that is now an alias for remotes::install_gitub

Using R 3.6.0
-- 
Mark Mendel
Director of Software Development
MEI Research, Ltd
6016 Schaefer Rd
Edina, MN 55436
mmendel using meinergy.com <jmoon using meinergy.com>
(651) 485-9 <%28952%29%20373-1636>369
http://meinergy.com/

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