Adding support for use with Go modules

[pafuent]

Previous to this patch if you try to run the test of go-git in a Go
modules environment, the tests fail due to go-git-fixtures package is
not found. In order to found the package on this environment, the
build.Context.Import function is leveraged.

Signed-off-by: Pablo Andres Fuente pablo.andres.fuente@gmail.com

[pafuent]

Also I'm wondering if the whole Init() function could be replaced by the code that I added. Theoretically the Import() function should find the correct go-git-fixtures package location, without the need of finding the vendor directories for example.

[dweomer]

I am looking into writing a test for a PR against src-d/go-git#1090 and ran into many errors with fixtures because I'm not using a GOPATH because GO111MODULES=on.

Besides a seemingly unrelated panic in gopkg.in/src-d/go-git.v4/plumbing/format/index IndexSuite.TestDecodeEndOfIndexEntry this worked for me by adding a replace directive in my local go-git go.mod:

replace gopkg.in/src-d/go-git-fixtures.v3 => github.com/pafuent/go-git-fixtures add_support_for_go_modules

/cc @mcuadros