basic::ImplRepository: use SolarMutex instead of own mutex

The locking strategy in the basic module is totally unclear to me, there
does not appear to be a dedicated mutex for the core stuff callable via
StarBASIC, just a bunch of SolarMutexGuards at random locations;
let's try to use SolarMutex at the UNO entry points...

Change-Id: Ia9c45fdcfb5ffd0a4acc77ef5d2fabfb8743ad38
basic: use SolarMutex to lock SfxLibraryContainer

Originally this used both SolarMutex and an own mutex, then a deadlock
was resolved in 2fe6a4a34b38c05e252c71f4d2f18e4a90e61b29 by not locking
SolarMutex.

Since the class will call event listeners without dropping the mutex
e.g. in insertNoCheck(), using the SolarMutex appears better anyway.

With this, installing a BASIC extension no longer triggers SolarMutex
asserts in SfxBroadcaster.

Change-Id: Ib9a2ee491ef53b1a53855af0fc22e863c5e7cb91
SwPostItMgr::RemoveItem(): AddressSanitizer: heap-use-after-free

Calling std::list::remove() iterates over all elements...

Change-Id: I46d00217911c1863f70d3ba72dc7099411714471