Add "no command line variant" for source-repository-package.

[philderbeast]

Adds a note that source-repository-package is not for the command line.

$ cabal build solver-benchmarks --source-repository-package=https://github.com/haskell/vector
Error: cabal: unrecognized 'build' option
`--source-repository-package=https://github.com/haskell/vector'

Also shows ".cabal file" as ".cabal file" and moves this paragraph up so that it is placed before the note and before any code blocks in this section.

Template B: This PR does not modify behaviour or interface

E.g. the PR only touches documentation or tests, does refactorings, etc.

Include the following checklist in your PR:

• Patches conform to the coding conventions.
• Is this a PR that fixes CI? If so, it will need to be backported to older cabal release branches (ask maintainers for directions).

[philderbeast]

Label merge+no rebase is necessary when the pull request is from an organisation.