Reworking value source API

[meatballhat]

What type of PR is this?

• cleanup
• feature

What this PR does / why we need it:

Reworking the "flag value source" API in prep for ensuring urfave/cli-altsrc can be easily used with urfave/cli/v3 by:

• reducing the surface area by keeping the environment variable and file implementations private
• generalizing to "value source" in name
• using the stdlib fmt.Stringer and fmt.GoStringer interfaces
• switching the value source interface to use Lookup to mimic stdlib semantics such as os.Lookup

Connected to #1531

[dearchap]

@meatballhat When you say "reintegration with altsrc" do you mean just have the API compatible with altsrc or actually pulling the alstrc codebase back into this repo ?

[meatballhat]

@meatballhat When you say "reintegration with altsrc" do you mean just have the API compatible with altsrc or actually pulling the alstrc codebase back into this repo ?

@dearchap I definitely do not mean bringing the altsrc codebase back here 😅 I'll update the description 👍🏼