Replace bst build --all with bst build --deps all

Based on an IRC discussion, it would make sense to bring bst build in line with other commands and replace the existing --all option with the usual --deps option.

The default value would be run to build the targets and their runtime dependencies, matching the current default. Specifying --deps all will build the targets and all their dependencies incl. build-only dependencies, matching the current behavior of --all.

--deps build could be supported as well to build the direct build dependencies of the targets and their runtime dependencies (targets themselves will not be built).

For both --deps run and --deps build build-only dependencies will implicitly be built if needed. This also matches the current default behavior.

For the corresponding user config file option, see !1469 (merged).