Support configuring spago file/target

[nwolverson]

Currently spago.dhall is hardwired to

• Detect PureScript "projects" (as one of several options)
• Serve as the config for spago sources (by virtue of being the default)

The build command (default spago build) is configurable but spago sources is not.

A current pattern for supporting multiple build targets (build, test, etc) is adding multiple spago.dhall files with an import/deps override, which can lead to e.g. test sources not being picked up as source globs.

HOWEVER with the registry project spago is moving to add a concept of build targets, so we should probably move directly to supporting that when it arrives.

Workaround in the meantime (probably): set eg

PURS_IDE_SOURCES=`spago -x test.dhall sources`