Exclude _build from ALL_MODULES

[jsoriano]

Rewrite filtering of files when discovering modules to use find options and exclude _build directory.

Including _build makes some targets to fail if the directory exists:

$ make gotest
make[1]: Entering directory '/home/jaime/gocode/src/github.com/elastic/opentelemetry-collector-components'
Running target 'test' in module '_build'
make -C _build test
make[2]: Entering directory '/home/jaime/gocode/src/github.com/elastic/opentelemetry-collector-components/_build'
make[2]: *** No rule to make target 'test'.  Stop.
make[2]: Leaving directory '/home/jaime/gocode/src/github.com/elastic/opentelemetry-collector-components/_build'
make[1]: *** [Makefile:25: _build] Error 2
make[1]: Leaving directory '/home/jaime/gocode/src/github.com/elastic/opentelemetry-collector-components'
make: *** [Makefile:37: gotest] Error 2