Code cleanup and reorganization

[jeff-cohere]

This PR is an attempt to clean things up a bit. I doubt it's perfect, but it seems like a structural improvement. Highlights:

• The core package has been eliminated, and the Database and Endpoint interfaces have been moved to the databases and endpoints packages. Meanwhile, the Frictionless data types have been moved to their own frictionless package.
• Some unwinding of dependencies between packages has been performed.
• The separate and almost-morally-equivalent Database and Endpoint test fixtures used by the tasks and services unit tests have been consolidated in the dtstest package. I hope their use makes some more sense than what they replace.
• I've made a cursory attempt to create some meaningful error types for error handling, and I've tried to make log messages a bit more uniform. Probably both are a work in progress.

Closes #42 Closes #43 Closes #44 Closes #46

[codecov[bot]]

Codecov Report

Attention: 36 lines in your changes are missing coverage. Please review.

Comparison is base (1e7f10a) 62.72% compared to head (3c6d9fd) 61.40%. Report is 2 commits behind head on main.

:exclamation: Current head 3c6d9fd differs from pull request most recent head 8be84b8. Consider uploading reports for the commit 8be84b8 to get more accurate results

Files	Patch %	Lines
tasks/tasks.go	52.00%	11 Missing and 1 partial :warning:
databases/jdp/database.go	68.18%	7 Missing :warning:
databases/databases.go	16.66%	5 Missing :warning:
endpoints/local/endpoint.go	68.75%	5 Missing :warning:
endpoints/globus/endpoint.go	72.72%	3 Missing :warning:
services/prototype.go	72.72%	3 Missing :warning:
endpoints/endpoints.go	50.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #47 +/- ## ========================================== - Coverage 62.72% 61.40% -1.33% ========================================== Files 12 12 Lines 2015 2026 +11 ========================================== - Hits 1264 1244 -20 - Misses 575 603 +28 - Partials 176 179 +3 ```

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