remove vendor prefix when storing packages

[davcamer]

This standardizes vendored packages so that they can be found using
the string from another package's .Imports list.

This addresses the issues I described in #10.

I believe vendor/ is a safe token to split on, based on this passage in the documentation:

Code below a directory named "vendor" is importable only by code in the directory tree rooted
at the parent of "vendor", and only using an import path that omits the prefix up to and including
the vendor element.

[kisielk]

Is something wrong with the GitHub diff, or with the commit? It looks like lines were only added but the old ones were not deleted.

[davcamer]

That was a problem with the commit. Sorry about that.