Forward -testfilter to nogo and fix failure in case of no srcs (baz…

…el-contrib#4075)

**What type of PR is this?**

Bug fix

**What does this PR do? Why is it needed?**

Without this, nogo runs on the external test sources when compiling the
internal test library, which can result in missing deps errors and is
also wasteful. Since filtering out files made certain nogo actions run
on no files, also fix a bug that affects this situation by writing out
an empty log in addition to an empty facts file.

The code that checks imports and builds the importcfg was shared between
`compilepkg` and `nogo` and is now extracted into a common method. Along
the way, have it output the file into the working directory, which
simplifies cleanup, makes the file easier to find and avoids writing
files unknown to Bazel into the output directory.

Also removes some unused test files.

**Which issues(s) does this PR fix?**

Fixes bazel-contrib#4062
Fixes bazel-contrib#4070
Fixes bazel-contrib#4073

**Other notes for review**

go/private/actions/archive.bzl

@@ -12,10 +12,6 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-load(
-    "//go/private:context.bzl",
-    "get_nogo",
-)
 load(
     "//go/private:context.bzl",
     "get_nogo",