Reorganize plugin packages.

[winder]

Summary

Nonfunctional changes, move all plugin packages under a common conduit package.

While moving things around I noticed that we have a lot of things named Metadata and Config. I'd like to find some better names and places for these things so that there are fewer things to pick from when implementing a new plugin.

Test Plan

CI

[codecov]

Codecov Report

Merging #1317 (1a890db) into conduit (9214b07) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           conduit    #1317   +/-   ##
========================================
  Coverage    60.68%   60.68%           
========================================
  Files           76       76           
  Lines        10913    10913           
========================================
  Hits          6623     6623           
  Misses        3703     3703           
  Partials       587      587           

Impacted Files	Coverage Δ
conduit/config.go	85.71% <ø> (ø)
conduit/pipeline/metadata.go	100.00% <ø> (ø)
conduit/pipeline/pipeline.go	65.67% <ø> (ø)
...duit/plugins/exporters/example/example_exporter.go	80.00% <ø> (ø)
conduit/plugins/exporters/exporter_factory.go	100.00% <ø> (ø)
...duit/plugins/exporters/filewriter/file_exporter.go	83.33% <ø> (ø)
conduit/plugins/exporters/noop/noop_exporter.go	70.83% <ø> (ø)
...lugins/exporters/postgresql/postgresql_exporter.go	80.23% <ø> (ø)
conduit/plugins/exporters/postgresql/util/prune.go	75.00% <ø> (ø)
conduit/plugins/importers/algod/algod_importer.go	78.02% <ø> (ø)
... and 16 more

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