Use mount instead of preload for tagfiles

[anutosh491]

Description

Please include a summary of changes, motivation and context for this PR.

Fixes #305
Closes #295

As discussed on the issue

1. xcpp.data should only be responsible for standard headers or resource dir headers
2. As tagfiles for any 3rd party lib would be expected to be picked up from the prefix, we should do the same for the tagfiles we provide.

Type of change

Please tick all options which are relevant.

• Bug fix
• New feature
• Added/removed dependencies
• Required documentation updates

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.78%. Comparing base (7ecb686) to head (11ef626).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #336   +/-   ##
=======================================
  Coverage   81.78%   81.78%           
=======================================
  Files          20       20           
  Lines         950      950           
  Branches       87       87           
=======================================
  Hits          777      777           
  Misses        173      173           

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[anutosh491]

Should probably go in only after the pr using the shared build for xeus.

[anutosh491]

cc @SylvainCorlay , this should be ready.