Support editable/develop installs of local build script dependencies

[NiklasRosenstein]

Currently, local dependencies (such as somepkg @ ../somepkg) can only be installed immediately. This makes development of a Kraken extension difficult in some situations. After modifying files in the source package, a reinstall needs to be triggered with krakenw --upgrade --incremental.

It would be great if we could install the local dependency in editable mode instead (e.g. Pip's -e option).

As a side note: We should add more emphasis on the warning after install (e.g. with --upgrade --incremental) if a Kraken lock file is present but it is outdated. Otherwise, the next krakenw run will install again whatever is in the lock file, and not what the buildscript() definition specifies.