[workspace] Clean up NLopt vendoring patches

[jwnimmer-tri]

Rename "vendoring.patch" to the more Drake-conventional "vendor.patch".

Instead of patching extern 'C' into the external code, adjust the BUILD file to distinguish between C++ code and C code and only run the vendor_cxx tool on C++ code.

Amends #17288.
Towards #17231.

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+@SeanCurtis-TRI how do you feel about both feature and platform review? Otherwise we can tag Rico for feature.

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This change is 

[SeanCurtis-TRI]

X2

Reviewed 4 of 4 files at r1, all commit messages.
Reviewable status: 2 unresolved discussions

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tools/workspace/nlopt_internal/package.BUILD.bazel line 173 at r1 (raw file):

_SRCS_STOGO_CABI = [
    "src/algs/stogo/stogo.cc",

BTW I was surprised to see a .cc file located in a CABI group. I had to look at the code to see why. For simple pattern matchers like myself, it might be worth making a note about why this counterintuitive pattern is nevertheless correct ("C-callable front-end" definition).

(Same on the AGS CABI below.)

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tools/workspace/nlopt_internal/patches/vendor.patch line 18 at r1 (raw file):

 // to the BFGS code below
 #if 0
-#include "nlopt.h"

BTW What's the value of removing a line guarded by #if 0?

[jwnimmer-tri]

Reviewable status: 1 unresolved discussion

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tools/workspace/nlopt_internal/package.BUILD.bazel line 173 at r1 (raw file):

Previously, SeanCurtis-TRI (Sean Curtis) wrote…

BTW I was surprised to see a .cc file located in a CABI group. I had to look at the code to see why. For simple pattern matchers like myself, it might be worth making a note about why this counterintuitive pattern is nevertheless correct ("C-callable front-end" definition).

(Same on the AGS CABI below.)

Added comments.

Also this one had both CABI symbols as well as what should have been a file-local class. I also added a patch file to mark it as such.

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tools/workspace/nlopt_internal/patches/vendor.patch line 18 at r1 (raw file):

Previously, SeanCurtis-TRI (Sean Curtis) wrote…

BTW What's the value of removing a line guarded by #if 0?

The vendor_cxx tool has some rough edges. An include statement here would cause the inline namespace to cross the #if 0 block.

[SeanCurtis-TRI]

Reviewed 3 of 3 files at r2, all commit messages.
Reviewable status: complete! all discussions resolved, LGTM from assignee SeanCurtis-TRI(platform)

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tools/workspace/nlopt_internal/patches/vendor.patch line 18 at r1 (raw file):

Previously, jwnimmer-tri (Jeremy Nimmer) wrote…

The vendor_cxx tool has some rough edges. An include statement here would cause the inline namespace to cross the #if 0 block.

Ahhhh.