[Rust] Can we get rid of some Python dependencies in all Rust builds

[Miro Hrončok]

Hello Rust folks,

I see that all Rust crate packages require rust-packaging which in turn 
requires python3-rust2rpm. That brings in:

  - Python (obviously)
  - python-jinja2
  - python-requests
  - python-tqdm

I always forget about this when I try to do some impact checks in Copr. 
Currently, I am rebuilding all (transitive) build-dependents of 
python-markupsafe in Fedora 35 and 36 due to a requested update. This includes 
~2k rust packages that could probably be skipped, becasue markupsafe is 
required by jinja2.

Do all rust packages indeed need all 3 listed Python libs on runtime?

As far as I understand, Python is used in the dependency generators trough the 
cargo-inspector entry-point. So, let's live with that. jinja2, requests and 
tqdm all seem to only by imported from the rust2rpm entrypoint (rust2rpm.__main__).

Can we please get rid of this dependency? It could be done on several levels:

  1) upstream pyp2rpm, either or:
    a) split the project into inspector, rust2rpm and common
       -- I consider this tedious
    b) make rust2rpm CLI a Python extra
       -- I consider this bad UX, since the upstream project *is rust2rpm*

  2) downstream rust-pacakging, all of the following:
    - move crate-inspector bits to rust-pacakging
    - stop requiring python3-rust2rpm from rust-pacakging
    - use modern Python macros to install the entry_point via pip
      (this is needed to be able to run ti without the metadata installed)

I have a proof of concept for (2) but it requires some small adjustment 
upstream -- the inspector module must not import form __main__ (that is very 
bad habit): https://src.fedoraproject.org/rpms/rust-packaging/pull-request/3

WDYT?
-- 
Miro Hrončok
-- 
Phone: [phone]
IRC: mhroncok