Refactor internals

[vladimyr]

This PR brings in bunch of internal changes:

1. stop caculating defaultNpmPrefix eagerly
   That's pretty much self explanatory because we might not need it in case prefix is being overridden by env or user config.
2. determining defaultNpmPrefix from process.execPath exclusively
   In all other cases we should rely on found config file (user/global) and value written inside.
3. moving %APPDATA% (windows) and Homebrew related stuff to getGlobalNpmrc
   This is not determining global npm prefix but rather determining location of global npmrc config which should then be used to determine global prefix.

To summarize:

1. if it is coming from env it is a envPrefix
2. if it is coming from user config it is a homePrefix determined from homedir's npmrc
3. if it is coming from global config (created during installation, like %APPDATA%/npm/etc/npmrc on Windows, or Homebrew's $(brew --prefix)/lib/node_modules/npm/npmrc) it is globalPrefix determined from globalNpmrc
4. if everything else fails default prefix is determined according to process.execPath

[sindresorhus]

@vladimyr Could use your thoughts in sindresorhus/resolve-global#3 (comment)

[sindresorhus]

// @TiagoDanin @boneskull Can you help review?

[vladimyr]

@vladimyr Could use your thoughts in sindresorhus/resolve-global#3 (comment)

I'll give my best although I'm not yarn user and my daily driver is macos.

[vladimyr]

Btw I did some experiments and figured out that npm has following order of preference when it comes to determining prefix value:

1. $npm_config_prefix env variable
2. prefix key in user config (home dir)
3. $PREFIX env variable
4. prefix key in global config (inside installation dir)
   In next commit I'll modify code to take that into account...

[sindresorhus]

@vladimyr Do you intend to tackle #13 or should I merge this now (if it's ready?) and leave that one for someone else?

[vladimyr]

@sindresorhus What I did here are mostly cosmetic changes ready for merge but it doesn't give us any benefit regarding #13 After investigating #13 I figured out that more thorough refactor is needed which I already started doing (locally) and I intend to finish this weekend. Sorry about late response, I'm not able to squeeze enough opensource time out of my schedule lately 🙃

[sindresorhus]

Ok. Great. I'll merge this then to keep your upcoming PR focused on fixing #13.