The getter.FileDetector was intended to be the final detector, only
converting a path to a file URL and returning a true in all cases. We
want to check for a local module before checking the registry so no
local modules that happen to match a registry module are broken.
Wrap the getter.FileDetector to check the module source's existence
before delegating the search to the registry.
Add a getter.Detector for detecting registry modules and looking up
the download location of the latest version. This is essentially a
temporary API until constraint solving is supported by the registry, as
then we'll have to supply the full set of known contraints to the
registry at once for resolution and we will fetch specific versions of
modules.
James Bardin