using path rather than filepath for URL-ish internal expressions

[sio4]

Internal expressions of paths should use path instead of path/filepath.

• The main issue was on htmlAutoRenderer.Render() which calls like ir.HTML("dir\file") in Windows.
• Added some more checks on testcases for buffalo.FS's Open().
• Functions assetPathFor() and loadManifest() are also fixed to make their jobs clear.
• Additionally, fixed returning error for ErrNotExist state of Open().

It should be fixed before #2259.

[sio4]

Hi @paganotoni, could you please take a look at this PR? it is required to fix #2259.

[paganotoni]

LGTM. Thanks @sio4

[sio4]

Thanks!