Preserve duplicate column names in query results

Queries returning duplicate column names - e.g. joins between tables
sharing column names - silently lost values because rows were built
with dict(zip(keys, row)), where the last duplicate wins.

Later occurrences are now renamed with a numeric suffix: id, id
becomes id, id_2 - skipping any suffix that would collide with a
real column in the same query.

The new utils.dedupe_keys() helper transforms the key list once per
query, so the per-row dict construction is unchanged and there is no
measurable performance impact.

Applied in Database.query() (including the PRAGMA and RETURNING
paths), Table.rows_where(), Table.search() and the CLI's JSON output.
CSV, TSV and table output keep the original duplicate headers.

Closes #624

sqlite_utils/cli.py

@@ -34,6 +34,7 @@ from .utils import (
     OperationalError,
     _compile_code,
     chunks,
+    dedupe_keys,
     file_progress,
     find_spatialite,
     flatten as _flatten,
@@ -3493,6 +3494,10 @@ FILE_COLUMNS = {
 
 
 def output_rows(iterator, headers, nl, arrays, json_cols):
+    # Duplicate column names would collide as dictionary keys, so rename
+    # later occurrences id, id -> id, id_2 - CSV and table output keep
+    # the original duplicate headers since they never build dictionaries
+    headers = dedupe_keys(headers)
     # We have to iterate two-at-a-time so we can know if we
     # should output a trailing comma or if we have reached
     # the last row.