Fix getFunctionIdsFromCallableArg to return the function id for array($this, 'foo') in params providers for example

[kkmuffme]

Required to get consistent types in e.g. params providers used in plugins.

[kkmuffme]

@weirdan merge please

[kkmuffme]

@weirdan this is an issue I encountered in an internal plugin we use - it's a custom params provider for a custom callable. Where would I put this params provider plugin in psalm's tests to test this?

[weirdan]

Wherever tests/Config/Plugin/Hook/FooMethodProvider.php is used I suppose

[kkmuffme]

@weirdan please merge