Exclude hidden files from logstream regex discovery

[jfzunigac]

This resolves a problem where Singer attempts to initialize logstreams for watermark files when the regex in the logstream configuration isn't left-bounded. For example if we have a regex: .*test.* and in the directory we have the following files:

.singer.my_logstream.test_001  <---- the watermark file
test_001 <---- the logfile

Singer will try to initialize logstreams for both files, resulting in a third watermark that tracks the watermark file:

.singer.my_logstream..singer.my_logstream.test_001 <---- unwanted watermark file
.singer.my_logstream.test_001  <---- watermark file
test_001 <---- logfile

In some cases when filenames are long, the processor won't be able to persist progress onto the unwanted watermark files due to filename too long exceptions.

Test Plan:

Added unit tests and tested in dev environment, as well as in kubernetes since its bound to happen more often there

[vahidhashemian]

Looks like the build failed because of the added unit test?