allow dirs that don't contain go files (fix #1306)

[travisjeffery]

Any feedback on how this should be behave is welcome. This is to fix #1306. The case I've hit is with repos with directories of just protobufs.

What version of Go (go version) and dep (git describe --tags) are you using??

Go 1.9.2 and dep on master.

What dep command did you run?

❯ dep ensure -add github.com/envoyproxy/data-plane-api

What did you expect to see?

❯ dep ensure -add github.com/envoyproxy/data-plane-api
Fetching sources...

"github.com/envoyproxy/data-plane-api" is not imported by your project, and has been temporarily added to Gopkg.lock and vendor/.
If you run "dep ensure" again before actually importing it, it will disappear from Gopkg.lock and vendor/.

With the repo vendored into vendor/.

What did you see instead?

Fetching sources...

Solving failure: No versions of github.com/envoyproxy/data-plane-api met constraints:
	master: Could not introduce github.com/envoyproxy/data-plane-api@master, as its subpackage github.com/envoyproxy/data-plane-api does not contain usable Go code (*build.NoGoError).. (Package is required by (root).)
	rds-redirect-code: Could not introduce github.com/envoyproxy/data-plane-api@rds-redirect-code, as its subpackage github.com/envoyproxy/data-plane-api does not contain usable Go code (*build.NoGoError).. (Package is required by (root).)
	rds_docs: Could not introduce github.com/envoyproxy/data-plane-api@rds_docs, as its subpackage github.com/envoyproxy/data-plane-api does not contain usable Go code (*build.NoGoError).. (Package is required by (root).)
	typo: Could not introduce github.com/envoyproxy/data-plane-api@typo, as its subpackage github.com/envoyproxy/data-plane-api does not contain usable Go code (*build.NoGoError).. (Package is required by (root).)

With this PR, the expected behavior is what you get.

[jhayotte]

LGTM

[sdboyer]

Sorry, but the change to facilitate this is going to need to be more involved. This approach basically just makes ListPackages() lie about whether or not there's Go code in a given directory.

I'll explain more over on #1306.