LegacySpecScanner - Degrade gracefully if we have temporary rebuilds during installation

LegacySpecScanner - Degrade gracefully if we have temporary rebuilds during installation

During the process of installing an extension, there are various flushes/rebuilds. Each of these flushes can lead to a (temporary) rebuild of the container.

In 5.54.0, a caching bug prevents these flushes from rebuilding the container promptly.

In 5.54.1 and 5.55.beta, the caching bug goes away -- and now it tries to rebuild the container (multiple times).
However, during the an early attempt, it fails: there is a mismatch between `getActiveModuleFiles()` (which is used by
`CRM_Api4_Services`/`LegacySpecScanner` to discover classes like `ImportSpecProvider`) and the active class-loader
(which is not available yet -- but it's supposed to load the source for `ImportSpecProvider`). This
means that `CRM_Api4_Services`/`LegacySpecScanner` may raise a class-not-found exception.

This patch basically makes `LegacySpecScanner` behave more like `ClassScanner` during the early parts
of bootstrap -- if a class is not loadable yet, then ignore it.

This should still work out in the end -- because there are multiple rebuilds, and the final disposition is based on the
final build.