Support vendor dir in IMPORT_URL location

[matihost]

https://github.com/sclorg/golang-container/blob/master/1.10/s2i/bin/assemble#L36
assumes that vendor directory is referenced from INSTALL_URL only.

However vendor dir can be any higher in directory structure.
Usually it is where IMPORT_URL points.

Fix proposal:

Here is my s2i assemble overridden script:
https://github.com/matihost/learning/blob/master/go/.s2i/bin/assemble

making project supporting https://github.com/golang-standards/project-layout structure build-able.

and usage:
https://github.com/matihost/learning/blob/master/go/Makefile#L67