Abbreviated targets

[kastiglione]

In my xchammer.yml, I listed an abbreviated label under targets, for example //Modules/User instead of //Modules/User:User. This resulted in no User-Bazel target, because it was looking for that label in the list of all targets, but the list of all targets uses a full label.

XCHammer should either support abbreviated labels, or generate an error when one is used.

For our project, I am wondering whether to use module paths instead of Bazel labels. For people new to bazel, Modules/User is natural but //Modules/User:User is foreign.

[kastiglione]

Closing in favor of #98