Feature request: add option to only build dependencies

[VRehnberg]

Add an option to eb to filter out the listed easyconfigs themself from the list of what to build.

Examples of wanted behaviour

Without flag:

$ eb -rD flex-2.6.4.eb
== Temporary log file in case of crash /dev/shm/eb-wwhadok4/easybuild-it4ro8d7.log
Dry run: printing build status of easyconfigs and dependencies
CFGS=/apps/easybuild-easyconfigs/easybuild/easyconfigs
 * [x] $CFGS/m/M4/M4-1.4.19.eb (module: M4/1.4.19)
 * [x] $CFGS/b/Bison/Bison-3.8.2.eb (module: Bison/3.8.2)
 * [x] $CFGS/f/flex/flex-2.6.4.eb (module: flex/2.6.4)
== Temporary log file(s) /dev/shm/eb-wwhadok4/easybuild-it4ro8d7.log* have been removed.
== Temporary directory /dev/shm/eb-wwhadok4 has been removed.

With flag:

$ eb -rD --only-deps flex-2.6.4.eb
== Temporary log file in case of crash /dev/shm/eb-wwhadok4/easybuild-it4ro8d7.log
Dry run: printing build status of easyconfigs and dependencies
CFGS=/apps/easybuild-easyconfigs/easybuild/easyconfigs
 * [x] $CFGS/m/M4/M4-1.4.19.eb (module: M4/1.4.19)
 * [x] $CFGS/b/Bison/Bison-3.8.2.eb (module: Bison/3.8.2)
== Temporary log file(s) /dev/shm/eb-wwhadok4/easybuild-it4ro8d7.log* have been removed.
== Temporary directory /dev/shm/eb-wwhadok4 has been removed.

Possible issues

• one of the listed easyconfigs is a dependency to another one?
• expected interaction with other flags like --copy-ec and PR-related ones?

[VRehnberg]

If one of the easyconfigs to be built with --only-deps is a dependency of another, I see a few feasible approaches:

1. throw an error
2. throw a warning and also filter out anything that would have been built with broken dependencies (but still as much as possible of the dependencies)
3. Do the filtering on a per easyconfig basis, i.e. the one listed easyconfig that is a dependency of another will be built despite the --only-deps flag

Note that no. 3 is what running --only-deps with an easystack file would do in all cases.