fix: packaging migration scripts

Running `./pants package ::` results in the following error. We included
the v3.5 migration script in `setup.py` and in the `pants_distribution`.
But, that script does not end in `.py`, so the `python_sources` target
did not "own" it. In order to correct this, we explicitly include the
extension-less binaries.

> ValueError: The explicit dependency
> `st2common/bin/migrations/v3.5/st2-migrate-db-dict-field-values` of
> the target at `st2common:st2common` does not provide enough address
> parameters to identify which parametrization of the dependency target
> should be used.
> Target `st2common/bin/migrations/v3.5:v3.5` can be addressed as:
>   * st2common/bin/migrations/v3.5:v3.5
>   * st2common/bin/migrations/v3.5/__init__.py
>   * st2common/bin/migrations/v3.5/st2_migrate_db_dict_field_values.py

In fixing this, I also noticed that we did not include the v3.8
migration script, so I marked that executable and included it as well.

Author: cognifloyd

st2common/BUILD

@@ -20,6 +20,7 @@ st2_component_python_distribution(
         "bin/st2-pack-download",
         "bin/st2-pack-setup-virtualenv",
         "bin/migrations/v3.5/st2-migrate-db-dict-field-values",
+        "bin/migrations/v3.8/st2-drop-st2exporter-marker-collections",
         "bin/st2-run-pack-tests:shell",
         "bin/st2ctl:shell",
         "bin/st2-self-check:shell",

st2common/bin/migrations/v3.5/BUILD

@@ -1,3 +1,5 @@
 # TODO: what to do about st2-migrate-db-dict-field-values ?
 # st2_migrate_db_dict_field_values.py is a symlink to st2-migrate-db-dict-field-values
-python_sources()
+python_sources(
+    sources=["*.py", "st2*"],
+)

st2common/bin/migrations/v3.8/BUILD

@@ -0,0 +1,3 @@
+python_sources(
+    sources=["st2*"],
+)