`vcpkg_fixup_pkgconfig()` doesn't handle multiple `Requries:` lines

[matwey]

Describe the bug
vcpkg_fixup_pkgconfig() doesn't support package configs with multiple Requires: lines. At least https://github.com/pkgconf/pkgconf implementation natively supports this syntax.
I have the following initial package config file:

prefix=/usr/local
libdir=/usr/local/lib64
includedir=/usr/local/include

Name: libweif
Version: 0.2.1
Requires: fftw3f
Requires: fftw3
Requires: fftw3l
Libs: -L${libdir} -lweif
Cflags: -I${includedir}

which is converted to

prefix=${pcfiledir}/../..
libdir=${prefix}/lib
includedir=${prefix}/include

Name: libweif
Version: 0.2.1

Libs: "-L${libdir}" -lweif
Requires: fftw3f
Cflags: "-I${includedir}"

by vcpkg_fixup_pkgconfig(). Note, that only first Requires: line was handled.

Expected behavior
All Requires: lines are present in the resulting .pc, or all requirements are merged in one line.

[dg0yt]

Regarding the original pc file, I wonder if multiple use of the same keyword is really something which is expected to work.

Even if it works with pkgconf, is it actually a feature covered by tests? (CC @kaniini)

There are at least three other, more or less relevant implementations: freedesktop.org pkg-config, u-config, and even CMake. (Did meson continue with the plan of adding a native parser?) Do they support this pattern?