Resolve "Workspace builds might not rebuild correctly when dependecies are updated"

Attempts to fix #215 (closed)

To resolve WIP we need to:

• Implement separate workspace class
• Write a test case that tests nested directories (to ensure diff_dirs works as expected)
• Write a test case that uses the in-between failed builds file log
• Do a bit of manual testing (man, these scenarios are hard to create)
• Decide what to do about the tarcache
• Merge !313 (merged)