feat: Speed up plugin source file detection

[mattmc3]

The try_source function could use some optimization. The current implementation profiles this way:

num  calls                time                       self            name
-----------------------------------------------------------------------------------
 1)    2          27.07    13.53   34.68%     27.07    13.53   34.68%  compaudit
 2)    9          32.95     3.66   42.22%     23.51     2.61   30.12%  _try_source
 3)    1          40.43    40.43   51.81%     13.37    13.37   17.13%  compinit

Using NULLGLOB pattern matching to populate a list of possible plugin init files and taking the first element of that list both simplifies the function, and also makes it faster.

num  calls                time                       self            name
-----------------------------------------------------------------------------------
 1)    2          27.29    13.64   39.04%     27.29    13.64   39.04%  compaudit
 2)    9          24.43     2.71   34.96%     14.20     1.58   20.32%  _new_try_source
 3)    1          40.83    40.83   58.42%     13.54    13.54   19.38%  compinit

If you are okay with this kind of advanced pattern matching, this PR also fixes #125 and supreceeds #126.

[wintermi]

Thanks @mattmc3

When I ran tests using zprof, I did not get the same performance improvement, though these changes simplify the code