[#21853] Adjust Go cross-compile to target entire package

[youngoli]

Without this change, cross-compilation fails on main packages with multiple files. #21853

This logic was already being applied in the test case but not the general case. Now it is just applied regardless. Instead of targeting a specific main.go file (ex. "foo/main.go") it will now target an entire directory (ex. "foo/.") and get all the other files needed.

Tested manually by running a pipeline with multiple files in the main package on Dataflow.

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[youngoli]

R: @lostluck

[codecov]

Codecov Report

Merging #21854 (35b68a6) into master (080f54a) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #21854   +/-   ##
=======================================
  Coverage   74.09%   74.09%           
=======================================
  Files         698      698           
  Lines       92503    92503           
=======================================
  Hits        68542    68542           
  Misses      22706    22706           
  Partials     1255     1255           

Flag	Coverage Δ
go	50.83% <ø> (ø)

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[damccorm]

LGTM - thanks!

[lostluck]

Possibly dumb question: This works even if there are test files for the main package?

[lostluck]

PRobably fine. The tool documentation explicitly says that for compiling a package, the _test.go files are ignored.