Some backends like `ib` don't have an obvious (nor practical) way to
easily download the entire symbology set available from all its mkt
venues. For such backends loading might require a non-std approach (like
using the contract search from some input mkt-key set) and can't be
expected to necessarily be supported out of the box. As such, allow
annotating a broker sub-pkg module with a `_no_symcache: bool = True`
attr which will make `open_symcache()` yield early with an empty
`SymbologyCache` instance for use by the caller to fill in the mkt and
assets tables in whatever ad-hoc way desired.
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