Resolve the source root the provisioning names

[estebanzimanyi]

The directory a MobilityDB checkout sits in belongs to whoever made it: the
provisioning action checks it out as _mobilitydb_src and hands the parse
MDB_SRC_ROOT, while the resolver's literal probe names _mobilitydb. The
resolver consults MDB_SRC_ROOT, so it answers over the checkout the
provisioning provides whatever that checkout is called, and still answers None
where no tree exists — the honest signal a caller degrades on.

Reading no source is silent in the output and expensive in it. The object model
reports errors: source-unavailable and carries no raised code, and
attach_type_relations locates no catalog and attaches nothing, so a consumer
receives a catalog with no typeRelations and cannot tell it from a complete
one. Against a provisioned checkout the same parse reports errors: scanned
with 685 functions carrying raised codes and 22 base types carrying their set,
span, span set and temporal relations, and the drift gate runs rather than
skipping.