Print warnings to stderr, not stdout

[hdgarrood]

At the moment I'm using spago sources to build a list of source globs which I then pass to purs directly. This mostly works great, except for the fact that some warnings get printed to stdout, which confuses things:

$ spago sources 2>/dev/null
WARNING: wasn't able to check if your package set file is frozen
.spago/aff/v5.1.1/src/**/*.purs
.spago/aff-coroutines/v7.0.0/src/**/*.purs
.spago/arrays/v5.3.0/src/**/*.purs
.spago/avar/v3.0.0/src/**/*.purs
.spago/bifunctors/v4.0.0/src/**/*.purs

This leads to a bunch of warnings such as:

> set -o noglob && purs compile $(npm run --silent sources:purs | tr '\n' ' ')

purs compile: No files found using pattern: WARNING:
purs compile: No files found using pattern: wasn't
purs compile: No files found using pattern: able
purs compile: No files found using pattern: to
purs compile: No files found using pattern: check
purs compile: No files found using pattern: if
purs compile: No files found using pattern: your
purs compile: No files found using pattern: package
purs compile: No files found using pattern: set
purs compile: No files found using pattern: file
purs compile: No files found using pattern: is
purs compile: No files found using pattern: frozen

Incidentally, once #173 is a thing I can probably just use that and not need to worry about this.

[f-f]

Good point. I think that since in some cases the output on stdout should be meaningful for machines too then the program should log everything on stderr.

I think this is also the easiest thing to do, since it's just about having echo print to stderr

[hdgarrood]

I've realised one potential issue with doing this, which is that the compiler currently prints json errors to stderr, so if you run spago build -- --json-errors then you'll get both the json errors and spago logging sent to the same stream.

I suppose one option is to also provide a --silent flag, like npm run --silent, which suppresses all output from spago itself so that you only get output from subcommands. I guess the compiler ought to print json errors to stdout really too, but that might be a bit awkward to change at this point.

[hdgarrood]

See also purescript/purescript#3672

[f-f]

Update: as we figured in #316, there is also the output from external commands, e.g. purs, getting in the way sometimes. So I think we should just output all logging to stderr (including piping output from external commands) except when printing actual "business" output (e.g. in case of spago sources, spago list-packages, spago run, etc)

I think we should just remove the current print facilities (echo and echoDebug) and replace them with output (that goes to stdout) and various "log" functions going to stderr: log, logDebug, logWarning, logError (these last two are the same as log except they prepend "WARNING: " etc to the log message. Eventually we could switch to some proper logging thing, e.g. I was considering RIO)