Fix README links in the README files

[aholmes]

This links to individual libraries point to src/<lib>/README.md. These are the wrong URLs everywhere except when navigating from within the monorepo. There are several other incorrect URLs in the individual library READMEs as well.

A temporary solution may be to change the URLs to the full HTTP URL for github, or for PyPI. A longer term solution may be to re-write the URLs during package building so the PyPI-uploaded packages point to the correct location, and the files in the repo remain unchanged and point within the repo.