Fix module path in go.mod and its references

[weitzj]

Currently the plugin cannot be built as a standalone postgres FDW as it differs from the standard way how the plugin main.go and go.mod have to look like.

go.mod and main.go should be fixed accordingly so that one can use the steampipe-postgres-fdw Makefile process to build standalong postgres-FDW like so:

# get makefile and standlone FDW toolchain
git clone https://github.com/turbot/steampipe-postgres-fdw.git
cd steampipe-postgres-fdw

# build the plugin
# currently does not work. Will work after this fix
make standalone plugin=gitlab plugin_github_url=github.com/theapsgroup/steampipe-plugin-gitlab

See:

• https://turbot-community.slack.com/archives/C044P668806/p1719409368775559