Add a "conditional require" call

What problem will this solve?

Provide a way to require a module if it exists. Return nil if the module does not exist (or does not meet any provided version requirements). This makes it easier to add conditional logic or overrides to third-party modules, which may or may not be installed.

What might be a solution?

local myModule = tryRequire('my-module')

if myModule then
    p.override(myModule, 'someFeature', function(base) ... end)
end

This call must:

• Return nil if the module does not exist
• If a version requirement is specified, return nil if the requirement is not met, without loading the module (which might have side effects)
• Any errors which occur while loading the module must be returned to the called (just as with require()

What other alternatives have you already considered?

#624 was a first attempt at solving this. However it needed to load the module to check the version, and could leave side effects if the version check failed, or the module failed to load. We'll need a way to check for the existence of a module, and to check its version, without actually loading it.

Anything else we should know?

Originally logged as #38; I decided it would be better to open a new issue rather than edit @TurkeyMan's description in place.

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Start by examining the existing require() behavior and the approach discussed in #624. Determine how module existence and version requirements can be checked without loading the module. Done means a conditional require returns nil for missing or incompatible modules, avoids loading them in those cases, and returns loading errors to the caller.