Panic upon invalid import involving an import map

Under Deno `1.23.3` and `1.24.0`, it is possible to produce a panic when running `deno check` given a particular invalid import configuration.

See a [minimal repro here](https://github.com/hen-x/deno-panic-repro).

Panic trace:

```
============================================================
Deno has panicked. This is a bug in Deno. Please report this
at https://github.com/denoland/deno/issues/new.
If you can reliably reproduce this panic, include the
reproduction steps and re-run with the RUST_BACKTRACE=1 env
var set and include the backtrace in your report.

Platform: macos aarch64
Version: 1.24.0
Args: ["/Users/henx/.deno/bin/deno", "check", "repro.ts"]

thread 'main' panicked at 'assertion failed: resolution_result.to_string().ends_with(\'/\')', /Users/ib/.cargo/registry/src/github.com-1ecc6299db9ec823/import_map-0.12.1/src/lib.rs:942:5
stack backtrace:
0: _rust_begin_unwind
1: core::panicking::panic_fmt
2: core::panicking::panic
3: import_map::resolve_imports_match
4: import_map::ImportMap::resolve
5: ::resolve
6: deno_graph::graph::resolve
7: deno_graph::graph::parse_module_from_ast
8: deno_graph::graph::parse_module
9: as core::future::future::Future>::poll
10: deno::proc_state::ProcState::prepare_module_load::{{closure}}
11: as core::future::future::Future>::poll
12: as core::future::future::Future>::poll
13: as core::future::future::Future>::poll
14: deno::main::{{closure}}
15: as core::future::future::Future>::poll
16: deno_runtime::tokio_util::run_local
17: deno::main
```