re-implement 'dm_learn_from_db()'

[TSchiefer]

It is the only function using legacy_new_dm() (with a data_model object as 2nd arg).

• Works without Define data model for INFORMATION_SCHEMA and expose it #342:

  • Define generic dm_meta() that dispatches on the dplyr source
  • Implement dm_meta() for SQLite
  • Implement dm_meta() for other databases

• Requires Define data model for INFORMATION_SCHEMA and expose it #342:

  • Implement dm_meta() for databases that have INFORMATION_SCHEMA
    • Learning from the database should be employing dbplyr/dm operations on that simpler dm instead of hard-coded SQL
  • Use dm_meta() in dm_from_src()

[krlmlr]

Calling tbl() is slow, we can avoid the slowness by calling tbl_sql(vars = ...) directly.

[krlmlr]

Now in #342.

[github-actions]

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