Only analyze files that are children of the targets in build()

[devdanzin]

When building, wily creates lists of target files to scan based on added and modified files from the git repository. It's possible to pass a path to wily build, but it won't currently avoid scanning files outside of the path.

This PR makes wily only scan files that are children of the chosen path during build. This is an improvement for building in repositories that contains large files that aren't interesting to analyze. The behavior remains unchanged if no path is passed to wily build.

This addresses a TODO in build.py, adapting some commented out code: https://github.com/tonybaloney/wily/blob/2590691d5d29a2872cc175bdb773b1dbf5fde202/src/wily/commands/build.py#L103