Fix pkg_deps_explain() for local package paths (#470)

[taekop]

pkg_deps_explain(".", "foobar") fails in the subprocess:

Caused by error in `adjlist[[wpkg]] <- deps1`:
! attempt to select more than one element in integerOneIndex

pkg_deps_explain_internal() finds the requested package with match(pkg, data$ref), but for a local path the solution's ref is local::., not .. match() returns NA, and adjlist[[NA]] <- ... is the error. The local::. spelling worked, which is the workaround in the issue.

When there is no exact match it now uses the row marked direct in the solution. If pkg matches ref as before, nothing changes.

The new tests/testthat/test-deps-explain.R uses two local packages and checks that "." gives the same paths as "local::.". It fails on main and passes here. I also compared 18 pkg_deps_explain() calls on main and on this branch (., ./, an absolute path, local::., upgrade = FALSE, dependencies = FALSE, repeated and self deps, and CRAN, cran::, standard::, versioned and GitHub refs). Everything that worked before gives the same output; the local path calls that errored now work. The full suite has the same failures on both builds (keyring, PPM SSO), minus the new test.

Closes #470