Support running executables which come from dependencies at the CLI

[Julian]

Given a dependency like LSpec which ships a lean_exe called lspec, it'd be nice to be able to run this binary from a repo depending on LSpec, after running lake build, without needing to manually specify the path to lspec as ./lean_modules/lspec/bin/lspec.

In e.g. npm, one has npx to run executables from node_modules without needing to manually specify them. Perhaps a similar separate executable or subcommand would be suitable for adding to lake.