This commit adds a new credential helper, NetrcCredentialHelper,
to retrieve credentials from a .netrc file. This replaces the
old .netrc authorization behaviour, which was done directly from
the `doWithAuth()` code.
Additionally, this commit moves all of the `.netrc` functionality
out of `lfsapi` and into `creds`. This was done both because
`creds` is now the logical place for the `.netrc` functionality,
and to prevent an import cycle between `creds` and `lfsapi`.
