allow us to override srcs in config using go tempate with the build rule

[goddenrich]

This is a potential approch to being able to deal with some more unusual build defs which generate go related targets. eg it allows us to define a srcs argument which the underlying build def uses when inputting to a known type. One example is

    mock_srcs_out = "%s.mock.go" % name
    mock_srcs = genrule(
        name = f"_{name}#mock_srcs",
        outs = [mock_srcs_out],
        cmd = f"$TOOLS_MOCKGEN -exec_only $TOOLS_REFLECT_PROGRAM -package {name} {package} {interfaces} > {mock_srcs_out}",
        labels = ["codegen"],
        test_only = True,
        tools = {
            "MOCKGEN": ["///third_party/go/go.uber.org_mock//mockgen"],
            "REFLECT_PROGRAM": [reflect_program],
        },
        deps = [
            reflect_program,
        ],
    )

    go_library(
        name = name,
        srcs = [mock_srcs],
        package = name,
        test_only = True,
        visibility = visibility,
        deps = [
            # Depend on the original lib since the mock files reference the original pkg.
            src_lib,
            "///third_party/go/go.uber.org_mock//gomock",
        ] + deps,
    )

and this allows us to define in config something which will make the srcs and therefore deps evaluate correctly

    "go_mock": {
      "providedDeps": ["///third_party/go/go.uber.org_mock//gomock"],
      "srcsRuleTemplate": "[\":_{{.Name}}#mock_srcs\"]"
    },

[Tatskaari]

LGTM though we should update the example to explain how/when to use this.