Use a more unique name for the cover variable (bazel-contrib#1764)

go/private/actions/cover.bzl

@@ -22,6 +22,10 @@ load(
     "structs",
 )
 
+def _sanitize(s):
+    """Replaces /, -, and . with _."""
+    return s.replace("/", "_").replace("-", "_").replace(".", "_")
+
 def emit_cover(go, source):
     """See go/toolchains.rst#cover for full documentation."""
 
@@ -40,8 +44,8 @@ def emit_cover(go, source):
         _, pkgpath = effective_importpath_pkgpath(source.library)
         srcname = pkgpath + "/" + orig.basename if pkgpath else orig.path
 
-        cover_var = "Cover_" + src.basename[:-3].replace("-", "_").replace(".", "_")
-        out = go.declare_file(go, path = cover_var, ext = ".cover.go")
+        cover_var = "Cover_%s_%s" % (_sanitize(pkgpath), _sanitize(src.basename[:-3]))
+        out = go.declare_file(go, path = "Cover_%s" % _sanitize(src.basename[:-3]), ext = ".cover.go")
         covered_src_map.pop(src, None)
         covered_src_map[out] = orig
         covered.append(out)