Fix bug in --exclude option

[taiki-e]

This fixes a bug described in the following comment:

cargo-llvm-cov/src/main.rs

Lines 237 to 239 in 16cd210

	// TODO: Currently, there is a problem that excluded crates are
	// incorrectly shown in the coverage report if they are path
	// dependencies of other crates.

(This PR's approach still incomplete in practice, but it should work in many cases.)

[taiki-e]

bors r+

[bors]

Build succeeded:

• ci