build: also build a "source-only" linux-packager-friendly tarball

We now distribute two tarballs, a "normal" one (the default, recommended for users),
and a new strictly source-only one (for Linux distro packagers).
The normal tarball, in addition to including everything from
the source-only one, also includes:
- compiled (`.mo`) locale files (in addition to source `.po` locale files)
- compiled (`_pb2.py`) protobuf files (in addition to source `.proto` files)
- the `packages/` folder containing source-only pure-python runtime dependencies

merges https://github.com/spesmilo/electrum/pull/7594
with follow-ups

electrum/paymentrequest.py

@@ -36,8 +36,7 @@ import aiohttp
 try:
     from . import paymentrequest_pb2 as pb2
 except ImportError:
-    # sudo apt-get install protobuf-compiler
-    sys.exit("Error: could not find paymentrequest_pb2.py. Create it with 'protoc --proto_path=electrum/ --python_out=electrum/ electrum/paymentrequest.proto'")
+    sys.exit("Error: could not find paymentrequest_pb2.py. Create it with 'contrib/generate_payreqpb2.sh'")
 
 from . import bitcoin, constants, ecc, util, transaction, x509, rsakey
 from .util import bh2u, bfh, make_aiohttp_session