[c#] cleaning up around "this" and composeMethodFullName

[xavierpinho]

Making sure we (a) use composeMethodFullName every time we build a methodFullName; (b) use Constants.This.

[xavierpinho]

Re-running jobs. Failed on php2cpg, cc @johannescoetzee :

[info] Detecting minified files
[info] - should skip minified files by name correctly
[info] - should skip minified files by content correctly
[info] - should parse all namespaces and move the files to the correct locations *** FAILED ***
[info]   0 was not greater than 0 (PhpDownloadDependenciesTest.scala:86)
[info] a PHP project with dependencies with a non-semver like version
[info] - should resolve and still get all namespaces