Added ability for wick reg pull to pull to the local dir (vs module hierarchy)

[jsoverson]

This PR adds a few options to the fetch options so that wick reg pull can optionally pull directly to the local directory vs. pulling everything to the standard wick module hierarchy, i.e.:

Instead of

$ wick reg pull common/azureblob:0.5.0
Pulled file:
- ./registry.candle.dev/common/azureblob/0.5.0/azureblob.wick
- ./registry.candle.dev/common/azureblob/0.5.0/azureblob-http-client.wick

You can add -F (for flatten) to get:

$ wick reg pull common/azureblob:0.5.0 -F
Pulled file:
- ./azureblob.wick
- ./azureblob-http-client.wick

This makes it easier to use existing registry components as templates to jump off of.