[#429] Remove alphanet.sh mentions

Description

Problem: there are still mentions of alphanet.sh in the code base.

Solution: change them all to tezos-client. Add a link to tezos-packaging to README to point to where one can get the executable.

Related issue(s)

Resolves #429 (closed)

✅ Checklist for your Merge Request

Related changes (conditional)

• Tests (see short guidelines)

  • If I added new functionality, I added tests covering it.
  • If I fixed a bug, I added a regression test to prevent the bug from silently reappearing again.

• Documentation

  • I checked whether I should update the docs and did so if necessary:
    • Root README and other README.md files
    • Haddock
    • docs/
  • I updated changelog files of all affected packages released to Hackage if my changes are externally visible.

Stylistic guide (mandatory)

• My commits comply with the following policy.
• My code complies with the style guide.