Satisfy HLint suggestions for isDigit and isAscii

[philderbeast]

See #9110. After #11093 merged, I started on suggestions with a two count so that these warnings are no longer ignored and will be picked up by our CI linting.

I stopped short of doing all of these as the suggested is digit and is ASCII changes lead me to another change to reuse isAsciiAlpha and because I'm unsure about when and why base functions should be added to module Distribution.Compat.Prelude.

cabal/Cabal-syntax/src/Distribution/Compat/Prelude.hs

Lines 6 to 11 in 9b26df8

	-- | This module does two things:
	--
	-- * Acts as a compatibility layer, like @base-compat@.
	--
	-- * Provides commonly used imports.
	module Distribution.Compat.Prelude

cabal/Cabal-syntax/src/Distribution/Compat/Prelude.hs

Lines 137 to 140 in 9b26df8

	, isDigit
	, isUpper
	, isAlpha
	, isAlphaNum

I also removed isNum, using isDigit instead in module Distribution.Client.VCS.

Each applied suggestion is an individual commit but I'll squash these if this pull request is approved.

---

• 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).