reorganize column collection init to be local

reorganize column collection init to be local

Reorganized the internals by which the `.c` collection on a
:class:`.FromClause` gets generated so that it is resilient against the
collection being accessed in concurrent fashion.   An example is creating a
:class:`.Alias` or :class:`.Subquery` and accessing it as a module level
variable.  This impacts the Oracle dialect which uses such module-level
global alias objects but is of general use as well.

Fixes: #12302
Change-Id: I30cb07c286affce24e2d85e49f9df5b787438d86