SqlBase::prepareQuery() should be called also on count

Problem/Motivation

Suppose that in a source plugin, extending from SqlBase, you want to override SqlBase::prepareQuery() and use that to add additional tags or meta-data to the query. You don't do that in ::query() method for some good reasons (for example you want to apply tags & meta-data only after the query was defined). If a hook_query_TAG_alter() that uses those tags adds a new condition then SqlBase::count() is lying.

For this reason, this is a blocker for #3069776: [PP-1]: SQL source plugins: allow defining conditions and join in migration yml, which introduces some query modifications in ::prepareQuery() method.

Proposed resolution

Make SqlBase::count() aware of SqlBase::prepareQuery().

Remaining tasks

Review / commit.

There are no User interface, API, or data model changes.