Improve PURL Component Search

### Current Behavior:
I have components that look like this:
```
pkg:maven/foo.bar.cat/xxxx@1.0.0?type=jar
pkg:maven/foo.bar.cat/yyyy@1.0.0?type=jar
pkg:maven/foo.bar.cats/xxxx@1.0.0?type=jar
```

It is not possible to search for `cat` without also getting hits on `cats` (that I do not want). The following search would be ideal (because it would exclude `cats`) but it does not work (returns 0 hits):

```
pkg:maven/foo.bar.cat/
```

Tweaking the search (just one character extra) works just fine... but excludes `pkg:maven/foo.bar.cat/yyyy@1.0.0?type=jar`

```
pkg:maven/foo.bar.cat/x
```

### Proposed Behavior:

Allow for PURL component search that can be truncated to the 2nd "/" character.

```
pkg:maven/foo.bar.cat/
```

This enhancement centres around PURL search because I would think that is easiest to fix. But, ideally, it would also useful to be more specific in (default) "coordinates" search by supporting specification of start and end of string. eg

```
^foo.bar.cat
foo.bar.cat$
^foo.bar.cat$
```
(something for a separate enhancement issue?)

The issue names no implementation files, tests, or entry points. Locate the PURL component search handling and its existing search tests, then verify that a query ending at the second slash matches foo.bar.cat while excluding foo.bar.cats and still matching both listed components.

Written by the indexing model from the issue text.