Show full path of `local_manifests` folder in deprecation warning

When a local_manifest.xml file is present, a deprecation warning
is printed telling the user to put local manifest files in the
`local_manifests` directory.

Include the full path to the `local_manifests` directory in the
warning, to reduce confusion about where the directory should be
located.  Also enclose the directory name in backticks.

Change-Id: I85710cfbd6e77fb2fa6b7b0ce66d77693ccd649f