Patch *.sdk paths from pkgConfig files to the current SDK

[fwcd]

This fixes #6439 by replacing all *.sdk paths obtained from pkgConfigs with the current SDK.

Motivation:

Using Homebrew-packaged libraries such as Cairo in a system library target on macOS currently causes build failures if the user compiles the Swift package with an Xcode toolchain (see swift-systemlib-demo and its CI builds for a minimal example), the build output often looks similar to this:

/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX13.0.sdk/usr/include/ffi/module.modulemap:1:8: error: redefinition of module 'FFI'
module FFI [system] [extern_c] {
       ^
/Library/Developer/CommandLineTools/SDKs/MacOSX13.sdk/usr/include/ffi/module.modulemap:1:8: note: previously defined here
module FFI [system] [extern_c] {

The problem is that Homebrew's pre-compiled libraries sometimes bake the Command Line Tools-paths (/Library/Developer/CommandLineTools/SDKs/...) into their pkgConfig files, which can clash with the current SDK's headers.

See #6439 for details.

Modifications:

As per @neonichu's suggestion in #6439 (comment), this patches the include (-I) and library (-L) paths parsed from pkgConfig files by replacing any *.sdk path with the current SDK.

Thus a flag such as

-I/Library/Developer/CommandLineTools/SDKs/MacOSX13.sdk/usr/include/ffi

would be mapped to

-I/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX13.3.sdk

when compiling with the macOS 13.3 SDK.

Result:

The swift-systemlib-demo package now compiles on macOS.

[MaxDesiatov]

@fwcd sorry about the delay with the review, would you mind resolving the conflicts?

[fwcd]

Sure, I've rebased it onto main!

[MaxDesiatov]

@swift-ci test

[MaxDesiatov]

@swift-ci test windows

[MaxDesiatov]

@swift-ci test windows