`os.findheader` support relative headerdirs

[mercury233]

What does this PR do?

Let os.findheader can support searching relative paths. Some libraries, by default, install header files within folders. For example, the header files for libfreetype-dev are located in: /usr/include/freetype2/freetype/*.h /usr/include/freetype2/ft2build.h

Previously, if a program included the header with #include <ft2build.h>, you had to use: os.findheader("freetype2/ft2build.h") .. "freetype2" to locate it.

Now you can simply use: os.findheader("ft2build.h", "freetype2") to achieve the same result.

How does this PR change Premake's behavior?

Nothing.

Anything else we should know?

Nothing.

Did you check all the boxes?

• [x] Focus on a single fix or feature; remove any unrelated formatting or code changes
• [x] Add unit tests showing fix or feature works; all tests pass
• [x] Mention any related issues (put closes #XXXX in comment to auto-close issue when PR is merged)
• [x] Follow our coding conventions
• [x] Minimize the number of commits
• [x] Align documentation to your changes

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[Jarod42]

How does this PR change Premake's behavior?

Nothing.

In fact behavior changes

os.findheader("header.h", "./submodules/library")

would no longer search from local directory.

Maybe creating another API would be cleaner, something like

function os.findheaderleaf(name, userpaths)
  local res = os.findheaderleaf(name, userpaths)
  if res then
    path.join(res, path.getdirectory(name))
  else
    return nil
  end
end