Return paths in PackageSourceMap deterministically (#2357)

in v1.220.0, specifically 28ea1a0, package source pathing was switched
over to a helper utility.  the `PackageSourceMap::paths` method looks
very similar to the logic before, but has a very subtle difference:

in order to avoid duplicate paths, it collects paths from each source
map's source files into a `HashSet` and then returns an iterator over
than (in contrast to before, which just presented a flat-mapped iterator
over all source files directly).

unfortunately, this means that iteration order is now random, which can
produce non-deterministic compilation when wit-parser is used as part of
the compilation macro, say for proc-macros like wasmtime-component macro.

it's... difficult to notice this in ordinary cargo compilation, but it
causes _extremely_ strange behavior in deterministic/caching build
tooling like buck2: given a series of deps `outer -> inner -> [something
that uses wasmtime-component-macro, like wasmtime-wasi]`, compiling
`outer` will cause rustc to reject `wasmtime-wasi` on a hash mismatch,
which will then surface as "unable to find crate "inner"` [^1]

fixing this is relatively easy: since our input iteration order is
deterministic, simply collecting into an `IndexMap` (already used
elsewhere in the file for this purpose) instead of a `HashMap` gives us
deterministic iteration order, and thus deterministic compilation.

[^1]: i know this happens when compiling `outer` as a static-pic for
library; i have not yet tested if it occurs in other compilation modes
like binaries or shared libs.

Author: DirectXMan12

crates/wit-parser/src/resolve.rs

@@ -168,7 +168,7 @@ impl PackageSourceMap {
             .iter()
             .flatten()
             .map(|path_buf| path_buf.as_ref())
-            .collect::<HashSet<&Path>>()
+            .collect::<IndexSet<&Path>>()
             .into_iter()
     }