extension: don't crash on malformed conflict/dependency ids

The conflicts and depends fields in the repository metadata are free
form text entered by extension authors and may contain values that ar

extension: don't crash on malformed conflict/dependency ids

The conflicts and depends fields in the repository metadata are free
form text entered by extension authors and may contain values that are
not valid extension ids, such as "sprintdoc template". Since 9af82229f
routed every Extension construction through the strict setBase()
validation, such a value made Extension::createFromId() throw an
uncaught RuntimeException while building the notices, taking down the
whole extension manager.

Skip entries that cannot be parsed into an extension id when checking
for missing dependencies and conflicts.

As an immeadiate fix this will also be fixed in the repository API to
avoid sending invalid extension IDs to the extension manager.

Fixes #4691

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