Support for non-aggregated projects

[satorg]

One more step towards #1481 resolution.
Although it probably does not resolve it completely – this PR only tackles on SBT builds.

Introduces a "sub-project" part for the buildRoots configuration option:

# It is possible to have multiple scala projects in a single repository. In that case the folders containing the projects (build.sbt folders)
# are specified using the buildRoots property. Also, an inner subproject for every build root can be specified after `:` (colon).
# It can be useful when there are subprojects in the build that are not aggregated into the root project. The subproject part cannot be empty.
# If there's no subproject part (the default) then the root project is assumed.
# Note that the paths used there are relative and if the repo directory itself also contains a build.sbt the dot can be used to specify it.
# Default: ["."]
buildRoots=[ ".", "subfolder/projectA", ".:subProjectX", "subfolder/projectB:subprojectY" ]

The : (colon) symbol is used as a separator between the relativePath and subProject parts in the configuration.

[satorg]

I'm not sure that non-aggregated (yet embedded) sub-projects are possible in all other build systems. So in order to add it there I may need additional time to find it out. But I'd appreciate your feedback on this part – let me know if I got it in the right way please. Thank you!