Repository rename tasks

[puerco]

This PR updates import paths, names and URIs across the code base following up the recent repository rename from slsa-source-poc to source-tool. This PR has the tasks required to migrate everything to the new name (except the items listed below):

Migration tasks:

This PR updates the following

• Main module renamed
• Linter configuration
• Import paths in all Go files
• Package names in protocol buffers definitions
• URLs in docs* (see below)

Not updated:

The follow locations have not been changed:

• Main README and GETTING_STARTED docs (new version is coming after the pilot program)
• URLs in provenance predicate types
• Old workflows we are deprecating for new ones in slsa-framework/source-actions
• URLs in the policy directory, being deprecated for those in slsa-framework/source-policies (see Deprecate and remove old actions and workflow #282 and Migrate existing repos to new actions #283)

Signed-off-by: Adolfo García Veytia (Puerco) [email protected]