all: build: fix sdkPath for relative @src.file / fix autocompletion with ZLS / IDEs (#661)

* all: build: fix sdkPath for relative @src.file

Prior to this commit, the build system heavily assumed that the result
`@src.file` would always be absolute, but this is no longer
guaranteed, likely due to there being no such thing as an "absolute
path" in WASI.

It appears that for normal invocations of `zig build`, it is safe to
assume that `@src.file` is absolute. However, when ZLS uses a custom
`build_runner.zig` to collect build configuration, `@src.file` is
actually relative to the current working directory, at least on my
system. For a while, this led to ZLS completions breaking entirely,
but presently it actually causes ZLS to crash!

The solution is not as simple as using relative `sdkPath` results
as-is, because the build system may attempt to resolve these paths
relative to build root, when the paths are actually relative to the
current working directory.

This leads to a sticky situation: the current working directory is a
runtime concept, but `@src.file` is resolved at compile time. However,
it appears that the build runner does not change current working
directory in between compilation and execution, so it is probably safe
to calculate `sdkPath` using runtime current working directory.

Still, this requires major changes with how `sdkPath` works, since
runtime computation and allocations are required. So pretty much
anything that relied on `sdkPath` being comptime-known has been
refactored in this commit.

The most severe result of this is that, for example, `gpu.pkg` can no
longer be a comptime-known constant: it has to be a runtime function
that takes a `*Builder` and returns a `Pkg`.

This commit deals with usages of `*.pkg` and `sdkPath` within Mach
itself, but projects that depend on Mach such as `mach-examples` will
almost certainly require changes as well.

* all: update README to reflect change in pkg usage

For details on updating your code to use this version, see: 88b1106953

Signed-off-by: Stephen Gutekanst <stephen@hexops.com>
Co-authored-by: Stephen Gutekanst <stephen@hexops.com>

libs/earcut/README.md

@@ -32,7 +32,7 @@ const earcut = @import("libs/mach-earcut/build.zig");
 
 pub fn build(b: *Builder) void {
     ...
-    exe.addPackage(earcut.pkg);
+    exe.addPackage(earcut.pkg(b));
 }
 ```