Optimize unit config searching code [5.3.0-B1]

The unit config is core of concept in In-Portal and it represents one unit (type of data, e.g. products) that can be operated upon. As for unit config searching code it:

• must be fast to find files quickly even in extensive directory structures
• skip folders, that are not useful (e.g. ".svn")
• return only folders, containing unit config files

I've found few places where we can speed up existing code (current time to find all unit configs: 0.0073s):

1. don't scan ".git" folder (we're already skipping ".svn" so this is natural to exclude this folder as well)
2. don't scan "install" folder, because in 5.3.x+ versions it would contain a lot of sub-directories, that won't have any unit configs in them
3. use "substr" instead of "preg_match" for skipping hidden folders

After above mentioned improvements the scan time dropped to 0.0068s.

Related Tasks

INP-1464 - Getting issue details... STATUS