zig build system: change target, compilation, and module APIs

Introduce the concept of "target query" and "resolved target". A target
query is what the user specifies, with some things left to default. A
resolved target has the default things discovered and populated.
In the future, std.zig.CrossTarget will be rename to std.Target.Query.
Introduces `std.Build.resolveTargetQuery` to get from one to the other.

The concept of `main_mod_path` is gone, no longer supported. You have to
put the root source file at the module root now.

* remove deprecated API
* update build.zig for the breaking API changes in this branch
* move std.Build.Step.Compile.BuildId to std.zig.BuildId
* add more options to std.Build.ExecutableOptions, std.Build.ObjectOptions,
  std.Build.SharedLibraryOptions, std.Build.StaticLibraryOptions, and
  std.Build.TestOptions.
* remove `std.Build.constructCMacro`. There is no use for this API.
* deprecate `std.Build.Step.Compile.defineCMacro`. Instead,
  `std.Build.Module.addCMacro` is provided.
  - remove `std.Build.Step.Compile.defineCMacroRaw`.
* deprecate `std.Build.Step.Compile.linkFrameworkNeeded`
  - use `std.Build.Module.linkFramework`
* deprecate `std.Build.Step.Compile.linkFrameworkWeak`
  - use `std.Build.Module.linkFramework`
* move more logic into `std.Build.Module`
* allow `target` and `optimize` to be `null` when creating a Module.
  Along with other fields, those unspecified options will be inherited
  from parent `Module` when inserted into an import table.
* the `target` field of `addExecutable` is now required. pass `b.host`
  to get the host target.

lib/std/Build/Step/WriteFile.zig

@@ -28,9 +28,6 @@ pub const File = struct {
     sub_path: []const u8,
     contents: Contents,
 
-    /// deprecated: use `getPath`
-    pub const getFileSource = getPath;
-
     pub fn getPath(self: *File) std.Build.LazyPath {
         return .{ .generated = &self.generated_file };
     }
@@ -126,10 +123,6 @@ pub fn addBytesToSource(wf: *WriteFile, bytes: []const u8, sub_path: []const u8)
     }) catch @panic("OOM");
 }
 
-pub const getFileSource = @compileError("Deprecated; use the return value from add()/addCopyFile(), or use files[i].getPath()");
-
-pub const getDirectorySource = getDirectory;
-
 /// Returns a `LazyPath` representing the base directory that contains all the
 /// files from this `WriteFile`.
 pub fn getDirectory(wf: *WriteFile) std.Build.LazyPath {