Perform some improvements to the componentlib library:

1) Rename some currently used defines that are very much "general" (ERROR, INSTALLED...) to some more "componentlib alternatives (COMPONENTLIB_ERROR...) to avoid collisions with other parts of Moodle.

2) Componentlib behaviour uses to download TWO files, the zip file itself and another one containing the md5 of the zip file. Currently only the zip file is downloaded using the standard download_file_content() function. Make that the md5 download to use that function too.

3) Obviously, update all the uses of the above 1 & 2 in core (environmental stuff and lang files).