matchers: use correct method for finding index in vector The path matcher has an optimization for when all paths are `rootfilesin:`. This optimization exists in both Python and Rust. However, the Rust implementation currently has a bug that makes it fail in most cases. The bug is that it `rfind()` where it was clearly intended to use `rposition()`. This patch fixes that and adds a test.

BasedOnStyle: LLVM
IndentWidth: 8
UseTab: ForIndentation
BreakBeforeBraces: Linux
AllowShortIfStatementsOnASingleLine: false
IndentCaseLabels: false
AllowShortBlocksOnASingleLine: false
AllowShortFunctionsOnASingleLine: false
IncludeCategories:
  - Regex:           '^<'
    Priority:        1
  - Regex:           '^"'
    Priority:        2