Adding SWH-compatible hash for source files of package versions

[mir-am]

This PR is a proof of concept to address fasten-project/fasten#102. A small plugin, swh-inserter, is created to add an SWH-compatible hash to the files in the metadata DB.

The plugin performs the following steps in order to update the metadata DB with SWH-compatible file hashes:

1. Given a package version from fasten.SourcesProvider.*, find its package version ID in the metadata DB.
2. From the metadata DB, retrieve the file paths for the package version ID in step 1.
3. For each file in step 2, do:
   3.1. read its source code file from the disk.
   3.2. compute an SWH-compatible hash for the source code in step 3.1.
   3.3 insert the obtained file hash into the metadata field of the files table by adding{'swh_checksum'}.

Testing

The implementation is tested using the DC. Also, for several random files, the computed hash in the DB is compared with git hash-object -w and they match.

[proksch]

After the pair programming session, I am happy with the current version.