reduce mem. usage of unused and update staticcheck

The primary improvement is in early clearing of
analyzed package's TypeInfo, facts, etc for
whole program analyzers (`unused`). Clear it when it
becomes unused and GC collects them early. Initially this
clearing was performed for all analyzers except `unused`.

Update staticcheck from v0.0.1-2019.2.3 to v0.0.1-2020.1.4

Also in this commit:
  * speed up loading packages from export data (2.5s -> 2.1s for std)
    by not using mutex for export data since it was allowed in
    x/tools#07722704da13
  * make an order of execution of linters stable
  * update renameio and robustio
  * use robustio in caching

Relates: #987, #994, #995, #1011

Author: jirfag

go.sum

@@ -385,6 +385,7 @@ golang.org/x/tools v0.0.0-20200422022333-3d57cf2e726e h1:3Dzrrxi54Io7Aoyb0PYLsI4
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 golang.org/x/tools v0.0.0-20200502202811-ed308ab3e770 h1:M9Fif0OxNji8w+HvmhVQ8KJtiZOsjU9RgslJGhn95XE=
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+golang.org/x/tools v0.0.0-20200502202811-ed308ab3e770 h1:M9Fif0OxNji8w+HvmhVQ8KJtiZOsjU9RgslJGhn95XE=
 golang.org/x/xerrors v0.0.0-20190717185122-a985d3407aa7 h1:9zdDQZ7Thm29KFXgAX/+yaf3eVbP7djjWp/dXAppNCc=
 golang.org/x/xerrors v0.0.0-20190717185122-a985d3407aa7/go.mod h1:I/5z698sn9Ka8TeJc9MKroUUfqBBauWjQqLJ2OPfmY0=
 golang.org/x/xerrors v0.0.0-20191011141410-1b5146add898 h1:/atklqdjdhuosWIl6AIbOeHJjicWYPqR9bpxqxYG2pA=