Honor the ${pcfiledir} entries in pkgconfig

[sergio-costas]

The ${pcfiledir} tag allows to make .pc files relocatables, by being replaced with the path where the .pc file is stored. In these cases, the "prefix=" entry must be left as-is, to ensure that it continues to work as expected.

It is complemented by canonical/craft-parts#977

Fix #5158

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.71%. Comparing base (654871d) to head (aa6ed53).
Report is 675 commits behind head on main.

❗ There is a different number of reports uploaded between BASE (654871d) and HEAD (aa6ed53). Click for more details.

HEAD has 1 upload less than BASE

Flag	BASE (654871d)	HEAD (aa6ed53)
	2	1

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5157      +/-   ##
==========================================
- Coverage   94.88%   89.71%   -5.17%     
==========================================
  Files         658      342     -316     
  Lines       55189    22642   -32547     
==========================================
- Hits        52364    20313   -32051     
+ Misses       2825     2329     -496     

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