Auto merge of #4974 - alexcrichton:less-vendor-verify, r=matklad

Be less strict about loading directory sources

Historically Cargo has been pretty strict about loading directory sources by
ensuring that everything inside was a crate. This was intended to protect
against accidental misconfiguration by failing loudly rather than silently
returning fewer crates.

This has caused a number of issues, however:

* #4969
* #4811
* #3899

so it seems like this is too aspirational of Cargo

Closes #4811
Closes #4969