go: Exclude vcs files when installing deps

Because our clones use the host git, on (say) Ubuntu 18.04, the local
git directories acquire perl scripts such as fsmonitor-watchman.sample.
During packaging, this leads to failures:

  ERROR: go-hsperfdata-1.0.3+gitAUTOINC+b58598ac84-r0 do_package_qa: QA Issue: /usr/lib/go/pkg/dep/sources/https---github.com-nsf-termbox--go/.git/hooks/fsmonitor-watchman.sample contained in package go-hsperfdata-staticdev requires /usr/bin/perl, but no providers found in RDEPENDS_go-hsperfdata-staticdev? [file-rdeps]
  ERROR: go-hsperfdata-1.0.3+gitAUTOINC+b58598ac84-r0 do_package_qa: QA run found fatal errors. Please consider fixing them.
  ERROR: go-hsperfdata-1.0.3+gitAUTOINC+b58598ac84-r0 do_package_qa:
  ERROR: go-hsperfdata-1.0.3+gitAUTOINC+b58598ac84-r0 do_package_qa: Function failed: do_package_qa

Signed-off-by: Alex Kiernan <alex.kiernan@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>