git-annex: create modules/annex

This moves the `annexObjectPath()` helper out of the tests and into a
dedicated sub-package as `annex.ContentLocation()`, and expands it with
`.Pointer()` (which validates using `git annex examinekey`),
`.IsAnnexed()` and `.Content()` to make it a more useful module.

The tests retain their own wrapper version of `ContentLocation()`
because I tried to follow close to the API modules/lfs uses, which in
terms of abstract `git.Blob` and `git.TreeEntry` objects, not in terms
of `repoPath string`s which are more convenient for the tests.

modules/git/tree_entry_gogit.go

@@ -89,6 +89,7 @@ func (te *TreeEntry) Blob() *Blob {
 
 	return &Blob{
 		ID:              ParseGogitHash(te.gogitTreeEntry.Hash),
+		repo:            te.ptree.repo,
 		gogitEncodedObj: encodedObj,
 		name:            te.Name(),
 	}