Update the scenarios to use vendored build dependencies

[zanieb]

Uses --find-links to discover vendored scenario build dependencies and allows us to use --index-url instead of --extra-index-url to avoid hitting the real PyPI in scenario tests.

[zanieb]

We prefer --find-links packages over the index packages, right?

[zanieb]

@BurntSushi perhaps you're more familiar with the Windows stack size work than me? Why would this cause stack overflows now?