This adds a new method PruneUnusedDependencies to a *FileBuilder. When creating a builder from an existing file, this can be used to remove its unused imports. If the file is mutated and elements removed such that some imports are no longer necessary, this enables removing those no-longer-used imports from the built descriptor.

This new method effectively removes any explicitly added imports. All imports in the built file will come from analyzing the actual referenced types in the file. Explicitly imported dependencies are typically used to provide knowledge of custom options (which cannot necessarily be found via basic examination of the builder hierarchy). But this patch preserves knowledge of custom options for descriptors created using builder.FromFile so that PruneUnusedDependencies can be safely used on these builders without resulting in inadvertently stripping custom options.

Resolves #504.