a job's input can specify the explicit input name

previously this would map to the resource's name. however, when resource
names change in the pipeline, that should not have to trickle down to
every single build.

this way a build can specify generic names (i.e. 'stemcell') that are
only needed for the build's scripting logic. a job then specifies the
mapping from these generic names to concrete resources in the pipeline
(i.e. 'aws-stemcell').

[#80072360]