ref: Move sourcelinks to symbolic-common

[loewenheim]

In preparation of using sourcelinks in sourcebundles, this extracts the sourcelink functionality from symbolic-ppdb and moves it to symbolic-common. It also refactors the implementation based on serde.

The JSON representation of a SourceLinkMappings struct looks like this:

{
    "C:\\src\\*":                   "http://MyDefaultDomain.com/src/*",
    "C:\\src\\fOO\\*":              "http://MyFooDomain.com/src/*",
    "C:\\src\\foo\\specific.txt":   "http://MySpecificFoodDomain.com/src/specific.txt",
    "C:\\src\\bar\\*":              "http://MyBarDomain.com/src/*",
    "C:\\src\\file.txt": "https://example.com/file.txt",
    "/home/user/src/*": "https://linux.com/*"
}

One downside of this is that it makes the serde dependency of symbolic-common non-optional. I don't know how big a deal this really is, but it may be that this code might more appropriately live in a different place.

[codecov[bot]]

Codecov Report

Merging #803 (e78fdf6) into master (09c291b) will decrease coverage by 0.23%. The diff coverage is 95.23%.

:exclamation: Current head e78fdf6 differs from pull request most recent head 3f90cb3. Consider uploading reports for the commit 3f90cb3 to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #803 +/- ## ========================================== - Coverage 75.03% 74.80% -0.23% ========================================== Files 71 63 -8 Lines 15630 15603 -27 ========================================== - Hits 11728 11672 -56 - Misses 3902 3931 +29 ```

[loewenheim]

The implementation has been simplified and the serde functionality removed.