Add support for `aux_files` option

[josmad]

Given files other.go and sample.go other.go

package mock_test

type Y interface{}

sample.go

package mock_test

type X interface {
  Y
}

type test interface {
  F(X)
}

The gomock stanza below fails with unknown embedded interface Y:

gomock(
    name = "private_mocks",
    out = "mocks.go",
    interfaces = [
        "test",
    ],
    library = ":go_default_library",
    package = "mock_test",
    source = "sample.go",
)

With this PR and modifying the stanza as below succeeds in generating the mock:

gomock(
    name = "private_mocks",
    out = "mocks.go",
    interfaces = [
        "test",
    ],
    library = ":go_default_library",
    package = "mock_test",
    source = "sample.go",
    aux_files = {
        "github.com/josmad/go_samples/mock_test": [
            "other.go",
        ],
    },
)

[jmhodges]

Hm, I'm looking at this again and is there no better way to get these file paths than assuming they're relative to the library's directory in the GOPATH?

[josmad]

I wanted to add an explicit dependency on a file within a go_library rule (or just the whole rule) to re-run mock generation if a listed aux_files changes. However, I was unsure how to do it (if it is even possible). If we can get the file paths of files in go_library rules it may be an option to change the attribute to reference go_library rules. Assuming that is possible I had two ideas:

• label_list and eagerly add all files in the referenced go_library.
• label_keyed_string_dict mapping from go_library rule to a file name listed in that rule.