"spack module tcl find -r <spec>" (and equivalents for other module
systems) was failing when a dependency was installed in an upstream
Spack instance. This updates the module index to handle locating
module files for upstream Spack installations (encapsulating the
logic in a new class called UpstreamModuleIndex); the updated index
handles the case where a Spack installation has multiple upstream
instances.
Note that if a module is not available locally but we are using the
local package, then we shouldn't use a module (i.e. if the package is
also installed upstream, and there is a module file for it, Spack
should not use that module). Likewise, if we are instance X using
upstreams Y and Z like X->Y->Z, and if we are using a package from
instance Y, then we should only use a module from instance Y. This
commit includes tests to check that this is handled properly.
406c791b88