Draft: Migrate the build system to hatch

This allows especially to install the sources in editable mode from a read-only directory. i.e. it allows to install it from sources in a docker environment where the source directory is mounter read-only, for example when running (docker based) integration tests using local sources.

We move static assets as proper package data (i.e. under swh/web/) for easier packaging.

Note: this is for reference, as a base for discussion about swh/meta#5083