Tighten warning for 'Requires.private' in simple pkg-config parser

[jabraham17]

Tightens up the logic for when we should warn about 'Requires.private' (and also 'Requires'). We only need to warn when the compiler/linker flags are not already handled by 'Cflags'/Libs.

Details:

Requires/Requires.private list what other pkg-config libraries are needed, so that pkg-config can pull in the need dependencies/flags. This ends up being a recursive operation (i.e. 'A' has 'Requires: B', so we must process 'B.pc'). But if we already have the information needed, this is unnecessary. So if we have 'Requires: B', but 'B' is already listed in 'Libs', then for our purposes we have satisfied the 'Requires.'

Our simple pkg-config parser is used only for bundled third-party libs, and we do not need to ability to recursively search other .pc files to satisfy the requirements. So this PR uses the above logic to avoid warning about Requires/Requires.private not being handled

This should resolve #26002

[Reviewed by @mppf and @jhh67]

[jhh67]

Very nice. Thanks for diagnosing the problem and coming up with a solution.

[mppf]

Trivial, but seems like really weird formatting. I would expect this:

if (static
    and d.get("Requires.private")
    and _pkgconfig_should_warn_for_requires(d, private=True)):

(But if there is some Python style convention that applies in this case, I wouldn't know about it).

[jabraham17]

I used black to format the code, but I think both formattings match the PEP8 style guide

[mppf]

Alright! Thanks for checking.