Simplify rFunctionExistsInModule source-module branch

[JTPetter]

The source-module (else) branch of rFunctionExistsInModule sourced every
.R file in the module's R/ directory into a temporary environment on each
call, just to check whether one function is defined.

This replaces it with a namespace lookup via getFromNamespace, wrapped to
return TRUE/FALSE. Per-call resolution dropped ~10x locally (≈0.23s → ≈0.02s)
and avoids repeatedly parsing/evaluating module source.

[Copilot]

Pull request overview

Replaces the source-file sourcing approach in rFunctionExistsInModule()'s source-module branch with a namespace lookup via utils::getFromNamespace(), wrapped in a small function_exists() helper. This avoids parsing/evaluating every .R file in the module on each call (~10x speedup locally).

Changes:

• Adds function_exists() helper that wraps utils::getFromNamespace() in tryCatch to return TRUE/FALSE.
• Rewrites the source-module branch of rFunctionExistsInModule() to call function_exists(moduleName, funName) instead of sourcing all R/*.R files into a temp env.