vpm: minor cleanup and refactor of install funcs

[ttytm]

I'm about to submit version-installs to allow to fix installations via vpm and dependencies to a tag. E.g.: v install https://github.com/vlang/vsl@v0.1.50 / v install vsl@0.1.50. Also some other fixes.

Some things could be cleaned up and solved more optimally in the current code. They are submitted separately before, as I think it would get a bit messy if they are mixed with other changes.

🤖 Generated by Copilot at 5ba25f3

Refactor and simplify vpm.v to support different vcs for installing modules. Extract a common install_module_from_vcs function and use a switch statement to handle different vcs types.

🤖 Generated by Copilot at 5ba25f3

• Extract install_module function to avoid duplication and improve readability and maintainability (link, link, link)
• Simplify and generalize vpm_install function to handle any supported vcs as the source argument (link, link)
• Simplify vpm_once_filter and vcs_used_in_dir functions by using built-in filter method on arrays (link, link)
• Rename vcs_key parameter to vcs in vpm_install_from_vcs function for consistency and clarity (link)
• Remove unnecessary comment line in vpm_install_from_vpm function (link)
• Simplify return value and use of vcs variable in vcs_used_in_dir function (link, link)

[ttytm]

In the end, the last commit breaks with keeping it a "minor cleanup" for this PR. Since the changes attempt to make a good improvement I hope the hassle reviewing it is okay.

[spytheman]

Sorry, but no. That hides too many opportunities for breaking stuff, and then when that happens, the only sensible thing would be to revert it, since too many things were changed at once :-| .

I suggest the following plan, done in separate PRs, instead:

1. Make the reorganization into a cmd/tools/vpm/ subfolder, WITHOUT any other changes,
   just move the vpm.v file there.

2. Add tests in that cmd/tools/vpm folder. There should be separate tests for installing a module with a github link directly like v install https://github.com/vlang/markdown, as well as installing a module through a vpm identifier, like v install nedpal.args, and also for installing v install pcre (short name, since it is under the vlang org).

3. Refactor stuff, and change things, keeping the existing tests running and adding more.

[ttytm]

Yep let's do so 👍