Allow specifying root directory for the project sources

For various reasons, I can't have Premake files in the root directory of the project, but have to put it into a subdirectory, let's call it build/premake. This means that even my simplest project definitions look like this:

project "foo"
  kind "StaticLib"
  symbols "On"

  local srcdir = "../../foo/"

  vpaths {
     ["Source Files"] = srcdir .. "**.cpp",
     ["Header Files"] = srcdir .. "**.h",
  }

  includedirs { srcdir .. "include" }

  files {
    srcdir .. "include/foo/bar.h",
    -- a dozen more of them...
    srcdir .. "src/foo/foo.cpp",
    -- and a dozen more of those...
  }

Having to write srcdir .. everywhere is aggravating.

My existing build system allows to do this instead

srcdir ../../foo; // This line makes all the paths relative to this directory.

library foo {
  includedirs = include;

  headers {
    include/foo/foo.h
    ...
  }

  sources {
    src/foo/foo.cpp
    ...
  }

}

and I find this much more convenient.

So I wonder if a similar option could be added to Premake? I'd be willing to implement it, but I'd appreciate any hints about how to do it because I am not sure how to get an option set at the current project level in translate() function which is where, I think, prepending of srcdir should be implemented.

Start by tracing the project-level path handling around the translate() function mentioned in the issue, and identify how current project options are made available there. Define the root-directory behavior for project paths, then verify that source, header, include, and vpath entries can omit the repeated prefix without changing existing relative-path behavior.