python: Re-land usage of source file path in toolchain picker (#31893)

This reverts commit 1e55e88c18.

Closes #ISSUE

Release Notes:

- Python toolchain selector now uses path to the closest pyproject.toml
as a basis for picking a toolchain. All files under the same
pyproject.toml (in filesystem hierarchy) will share a single virtual
environment. It is possible to have multiple Python virtual environments
selected for disjoint parts of the same project.

crates/proto/proto/toolchain.proto

@@ -23,6 +23,7 @@ message ListToolchainsResponse {
     repeated Toolchain toolchains = 1;
     bool has_values = 2;
     repeated ToolchainGroup groups = 3;
+    optional string relative_worktree_path = 4;
 }
 
 message ActivateToolchain {