fix external (non-rebar3) project compile

[aboroska]

Attempt to fix #1977

[ferd]

This essentially appears to turn off validating whether the project can be built at all. So while this might fix a rabbit build, it means that you could check out a totally invalid dependency (such as a Haskell project) and we would hardly give a warning about it.

I don't think this is acceptable.

[aboroska]

Yes, I agree. As I mentioned in #1977 that the check should be moved to after running the pre-hooks. I amended the PR with where the verification should be in my opinion. Left the error message as it was before.

Tested by removing the pre compile hook, so the following config:

{deps, [{rabbit_common, {git, "https://github.com/rabbitmq/rabbitmq-common.git", {ref, "9719670db30"}}}]}.

Now gives:

===> Compiling rabbit_common
===> {dep_app_not_found,<<"rabbit_common">>}

for rebar3 compile

[tsloughter]

I think we should probably allow for the dep to not be an OTP structured app until after pre-hooks.

One thing missing in this is a format_error function for when the dep_app_not_found error is thrown.