Invoke pip with --no-deps.

We want to make sure all our dependencies are specified in requirements.txt.
Fix some spelling errors.
Get the test server finding the right tree dirs, based on the instance dir.

This also represents a conceptual shift: a tree now does in target/trees/<tree_name>. dxr-build doesn't yet reflect this.
Load the instance-local config into Flask's config system.

Capitalize the config keys so we don't have to override any Flask stuff; Flask requires capitals.
Get the front page serving.

Flask says slashes here are platform-independent.
Make directory listings and ".html"-less source code URLs work.