Move watcher to internal package, clean up, add tests

[roblillack]

This change set:

• Moves the Watcher type to an internal package ./internal/watcher
• Adds two unit tests to said functionality
• Cleans up error handling of the watcher (return more errors more often)
• Drops anything from the watching code that was necessary to do compilation (eager watching, forced refresh)
• Removes functionality to have listener filter based on file/dir name (DiscerningListener)
• Removes unnecessary utility functions

The code uses wrapped errors, so minimum Go version will be 1.13 as soon as this is merged.

Ticket: #16

[coveralls]

Coverage decreased (-0.6%) to 59.897% when pulling b717104 on 16-hide-some-internal-data-structures into 60bd1e5 on master.