Change import of "tester" from relative path to full path

[anthonyfok]

This solves the "unexpected directory layout" error when go test is
called like so: go test github.com/ianbruene/go-difflib/difflib/...

This was discovered when packaging github.com/ianbruene/go-difflib as golang-github-ianbruene-go-difflib for Debian:

   dh_auto_test -O--builddirectory=_build -O--buildsystem=golang
        cd _build && go test -vet=off -v -p 4 github.com/ianbruene/go-difflib/difflib github.com/ianbruene/go-difflib/difflib/bytes github.com/ianbruene/go-difflib/difflib/tester
unexpected directory layout:
        import path: _/<<PKGBUILDDIR>>/_build/src/github.com/ianbruene/go-difflib/difflib/tester
        root: /<<PKGBUILDDIR>>/_build/src
        dir: /<<PKGBUILDDIR>>/_build/src/github.com/ianbruene/go-difflib/difflib/tester
        expand root: /<<PKGBUILDDIR>>/_build
        expand dir: /<<PKGBUILDDIR>>/_build/src/github.com/ianbruene/go-difflib/difflib/tester
        separator: /
unexpected directory layout:
        import path: _/<<PKGBUILDDIR>>/_build/src/github.com/ianbruene/go-difflib/difflib/tester
        root: /<<PKGBUILDDIR>>/_build/src
        dir: /<<PKGBUILDDIR>>/_build/src/github.com/ianbruene/go-difflib/difflib/tester
        expand root: /<<PKGBUILDDIR>>/_build
        expand dir: /<<PKGBUILDDIR>>/_build/src/github.com/ianbruene/go-difflib/difflib/tester
        separator: /
dh_auto_test: error: cd _build && go test -vet=off -v -p 4 github.com/ianbruene/go-difflib/difflib github.com/ianbruene/go-difflib/difflib/bytes github.com/ianbruene/go-difflib/difflib/tester returned exit code 1

I've already applied the patch for the Debian package (see https://salsa.debian.org/go-team/packages/golang-github-ianbruene-go-difflib/-/blob/debian/sid/debian/patches/01-fix-unexpected-directory-layout-error-during-go-test.patch) so it is OK on the Debian side.

Please feel free to merge this PR or not depending on your preference on relative import paths vs. full paths.
(I was reading the debate at golang/go#20883 and realized both sides have good arguments...)

Many thanks!

Cheers,
Anthony

[rcoreilly]

This is now a blocking bug on go 1.18 -- go mod tidy chokes on it. any chance you could apply this PR?

[Dorbmon]

Yes. Now it stops me from compiling it...