getdeps: make sure ManifestLoader never reloads manifests
getdeps: make sure ManifestLoader never reloads manifests

Summary:
In response to review feedback for D16477400 and D16477401, update
`ManifestLoader.load_all_manifests()` to only update its data for projects
that have not previously been loaded.  This helps ensure that code using a
single `ManifestLoader` object cannot have two in-memory `Manifest` objects
for the same project, and that existing data (such as project hashes) can't be
invalidated if a manifest is later loaded from updated on-disk data.

Reviewed By: pkaush

Differential Revision: D16586682

fbshipit-source-id: 50b1979ec55f2ad6901629cd852293a8f6ca903f

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly