ngclient: implement glob-like pattern matching

[sechkova]

Description of issue or feature request:

According to the recently updated version of the specification the shell style wildcard matching is glob-like, and therefore a path separator in a path should not be matched by a wildcard in the PATHPATTERN.

Current behavior:
A path separator in a path is matched by a wildcard in the PATHPATTERN (fnmatch.fnmatch)

Expected behavior:
A path separator in a path should not be matched by a wildcard in the PATHPATTERN.

[sechkova]

See related issue #1506

[sechkova]

Currently the implementation uses fnmatch from the fnmatch module which doesn't distinguish the filename separator ('/' on Unix) as a special character.
The recommended "path-aware" module by the python docs is the glob module. However glob.glob does it by accessing the filesystem and working relative to the current dir (or it needs an absolute path). This behaviour does not work in our case where we need to match relative paths:

• target_path: A target identifier that is a path-relative-URL string (https://url.spec.whatwg.org/#path-relative-url-string). Typically this is also the Unix file path of the eventually downloaded file.
• pathpattern

Internally glob seems to use fnmatch after having parsed the path segments, maybe implementing similar functionality but treating relative paths as strings, without file system access, could do the job.