refactor: lookup exec interpreter using toolchain resolution (#1997)

This changes the exec tools toolchain to use toolchain resolution to
find the interpreter.
The main benefit of this is it avoids having to duplicate specifying
where the interpreter
is. Instead, it is automatically found by toolchain resolution.
diff --git a/python/private/common/py_library.bzl b/python/private/common/py_library.bzl
index 977bdb3..673beed 100644
--- a/python/private/common/py_library.bzl
+++ b/python/private/common/py_library.bzl
@@ -64,6 +64,7 @@
     """
     check_native_allowed(ctx)
     direct_sources = filter_to_py_srcs(ctx.files.srcs)
+
     precompile_result = semantics.maybe_precompile(ctx, direct_sources)
     direct_pyc_files = depset(precompile_result.pyc_files)
     default_outputs = depset(precompile_result.keep_srcs, transitive = [direct_pyc_files])