Cache parent depMap

[guybedford]

When running loader.resolve('./x', 'y') we should be able to use the cached parent context depMap for already-loaded modules to ensure we cache resolutions fully, and never have to re-run the same normalization twice. This makes the depMap base-level, which may effectively make trace: true the default. This should be carefully measured for performance benefits.

[guybedford]

parentMetadata is now public, so that this can be internally cached by the loader implementation. Resolve itself needs to run predictably so we shouldn't have caching in the base loader itself as that would expose an unpredictable interface to loader hook implementors.