ingestion manifest creation for EVLA CAL ingestion only

Next step is refactoring to IngestionManifestIF and having EVLA CAL ingestion manifest implement it, but I wanted to get this out there for review.

apps/cli/executables/pexable/ingest_envoy/ingest_envoy/ingest.py

+""" The entrypoint (to be renamed from "main" at some point"""
+
+import sys
+from pathlib import Path
+
+from ingest_envoy.ingestion_manifest import IngestionManifest
+from ingest_envoy.utilities import ScienceProductType
+
+
+def main():
+    """
+    Takes a single command-line parameter: spool directory containing files to be ingested.
+
+    :return:
+    """
+
+    # parser handles command-line args; no validation needed here
+    staging_dir, ingestion_type, locator = sys.argv[1], sys.argv[2], sys.argv[3]
+
+    # nor here
+    source_dir = Path(staging_dir)
+
+    # ValueError will be thrown if ingestion_type isn't a known ScienceProductType
+    ingestion_type = ScienceProductType.from_str(ingestion_type)
+
+    # Launch the manifest builder
+    IngestionManifest(source_dir, ingestion_type, locator).create()