In a future commit, we're going to be accessing credential handling from
multiple packages. To avoid an import loop, move credential handling
into its own package. Update all the callers of the credential handling
code to use a qualified name.
Where there is a local variable called "creds", which would conflict
with our package name, rename it "cred" instead.
This brings support for SingleSignOn on windows using the default credentials of the currently logged in user if
an empty username and empty password is provided from gitcredentials. This plays well with the Git for windows
implementation which stores an empty username and password if it should use the default credentials.
