[analysis_server] Only replace full paths or start of paths followed by separators during normalization

It's possible that there are nested paths in LSP workspace folders, such as:

/foo
/fooBar
/bar/foo

We shouldn't just replace each path we were given, as it might replace the wrong parts of other parts. Instead, require the path to be surrounded by quotes, or prefixed by a quote and followed by the appropriate path separator.


I also tweaked the benchmark test a little to report the time for the first call separately from the others, since that one triggers the regex build.

On my machine, timings currently are:

```
Replacing 250 paths in payload of 2097152 bytes
Iteration #1, First: 43ms, Rest: 30ms
Iteration #2, First: 38ms, Rest: 30ms
Iteration #3, First: 33ms, Rest: 30ms
Iteration #4, First: 37ms, Rest: 30ms
Iteration #5, First: 31ms, Rest: 29ms
```

Change-Id: I75de1fb36a16636e0f50099cced1d5386318d589
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/491640
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Reviewed-by: Samuel Rawlins <srawlins@google.com>
Reviewed-by: Keerti Parthasarathy <keertip@google.com>