internal: support scanning path

[jroimartin]

This PR makes the following changes:

• Introduce the concept of "Lava asset types". Which are asset types
  that are only valid in the context of Lava. For instance, Path.
• Map Lava asset types to Vulcan asset types and provide tools to work
  with this mappings under a new package called assettype. For
  instance, Path maps to GitRepository.
• Add the method gitserver.*Server.AddPath, which allows to serve
  any file path as a Git repository. This Git repository contains a
  single commit that includes all files and directories under the
  specified path.
• Handle the Path asset type in the Lava engine.

This allows Lava users to scan any arbitrary path without the
limitations of a Git repository. For instance,

• All ignored and non-pushed files/changes are scanned.
• It is possible to scan a subtree within a repository.

[seilagamo]

LGTM. Remember to clean the commit before merging.