Add ability to create temporary assets, which are only available to other builders.

[jakemac53]

These can still be cached on disk, but should live in some hidden directory, not wherever the rest of the build outputs are written.

For example the angular transformer has many intermediate assets which it cleans up later on. These assets still can't be deleted by later builders, but that shouldn't be an issue.

[jakemac53]

This is probably best left up to the individual builders. They can choose where to output files, including inside of hidden folders etc.