fix(wkg): respect member manifests during fetch

[subotac]

Fixes #243.

When wkg fetch ran below a workspace root, it always selected workspace mode even when the current directory had its own wkg.toml. Use the member manifest as the fetch context in that case while preserving
workspace behavior when no local manifest exists.

The regression test verifies that fetching from a member creates the member lock file rather than a workspace lock file.

Testing

• cargo test -p wkg --test e2e fetch_from_member_manifest_uses_member_context --no-default-features
• cargo check -p wkg --tests --no-default-features
• cargo test -p wkg --bin wkg --no-default-features
• cargo fmt --all -- --check

[mkatychev]

Would this approach work if one is in a subdirectory of a member manfiest, for instance, will this work if we are in subdir?

./root
├── member
│   ├── subdir
│   │   └── file.wit
│   └── wkg.toml
└── wkg.toml

Manifest::load_root_workspace will keep walking up the path until it finds a manifest that is a workspace root.
It may be better to handle this using find_root_manifest_for_wd:

wasm-pkg-tools/crates/wasm-pkg-core/src/manifest/paths.rs

Lines 6 to 7 in 3d79978

	/// Find the first ancestor [`super::Manifest`] path for current working directory.
	pub(crate) fn find_root_manifest_for_wd(cwd: impl AsRef<Path>) -> Option<PathBuf> {